A Fly Carries Germs on Its Legs and a Mosquito Breeds Them Inside
Separate communicable from non-communicable disease, learn what endemic, epidemic, pandemic and sporadic actually describe, follow the air-borne and water-borne routes, and control the three vectors.
Why can you protect yourself from a fly by covering the food but not from a mosquito?
A housefly that has walked on filth and then settles on your food leaves germs behind. Cover the food and the problem is solved — the fly has nowhere to put them.
A mosquito cannot be dealt with that way. Covering food achieves nothing, because the mosquito does not deliver its germs onto food. It injects them, through your skin, when it bites.
And there is a deeper difference than where the germ is delivered. For the housefly the germ is simply a passenger — it sticks to the hairy legs and body, rides along, and gets wiped off wherever the fly next lands. The fly is a mechanical vector.
For the mosquito the germ is a resident. The malarial parasite actually develops inside the mosquito, completing part of its life cycle there before it can infect anybody at all. The mosquito is a biological vector.
So one insect is a vehicle and the other is a nursery, and the control measures follow directly: you stop a vehicle by denying it a destination, and you stop a nursery by destroying the place where the young are raised.
That is the shape of this whole chapter. Before you can prevent a disease you have to know how it travels — and before that, whether it travels at all, because many serious diseases do not spread from person to person in the first place.
This page covers the first part of the ICSE Class 9 Biology chapter on health and hygiene: communicable and non-communicable disease, the four terms describing how widely a disease occurs, the air-borne and water-borne routes, and the three vectors with their control.
A mosquito cannot be dealt with that way. Covering food achieves nothing, because the mosquito does not deliver its germs onto food. It injects them, through your skin, when it bites.
And there is a deeper difference than where the germ is delivered. For the housefly the germ is simply a passenger — it sticks to the hairy legs and body, rides along, and gets wiped off wherever the fly next lands. The fly is a mechanical vector.
For the mosquito the germ is a resident. The malarial parasite actually develops inside the mosquito, completing part of its life cycle there before it can infect anybody at all. The mosquito is a biological vector.
So one insect is a vehicle and the other is a nursery, and the control measures follow directly: you stop a vehicle by denying it a destination, and you stop a nursery by destroying the place where the young are raised.
That is the shape of this whole chapter. Before you can prevent a disease you have to know how it travels — and before that, whether it travels at all, because many serious diseases do not spread from person to person in the first place.
This page covers the first part of the ICSE Class 9 Biology chapter on health and hygiene: communicable and non-communicable disease, the four terms describing how widely a disease occurs, the air-borne and water-borne routes, and the three vectors with their control.
Which diseases spread from person to person and which do not?
A communicable disease is caused by a pathogen and spreads from an infected person to a healthy one; a non-communicable disease has no pathogen and does not spread at all.
Communicable, or infectious, diseases.
- Caused by pathogens — bacteria, viruses, protozoa, fungi or worms
- They spread from an infected person to a healthy one, either directly by contact or indirectly through air, water, food or a vector
- Examples: cholera, typhoid, tuberculosis, influenza, the common cold, measles, chicken pox, malaria, AIDS, ringworm, polio
Non-communicable, or non-infectious, diseases.
- Not caused by a pathogen, and they do not spread from person to person
- They arise from deficiency, heredity, degeneration, hormonal imbalance, allergy or the way a person lives
- Deficiency diseases: scurvy, rickets, anaemia, goitre, night blindness, kwashiorkor
- Hereditary diseases: haemophilia, colour blindness, sickle-cell anaemia
- Degenerative diseases: arthritis, cataract
- Hormonal disorders: diabetes, goitre
- Allergic conditions: asthma
- Others: cancer, high blood pressure, heart disease
Now the boundary case that catches people out. A non-communicable disease can still be extremely common in one area. Goitre is widespread wherever the soil and water are short of iodine, and whole villages may be affected.
That has nothing whatever to do with contagion. Everybody in the village has the same disease because everybody drinks the same iodine-poor water, not because they caught it from one another.
So "many people in one place have it" does not make a disease communicable. The test is whether it has a pathogen that passes from one person to another — and if it does not, then no amount of isolation, disinfection or quarantine will reduce it by a single case. What will reduce it is iodised salt, and that is why the distinction is practical rather than merely a matter of naming.
A second boundary worth knowing. Some conditions have more than one cause. Goitre is usually a deficiency disease and can also be hormonal; anaemia can come from iron deficiency, from blood loss, or from a hereditary defect. So a disease may appear in two of the lists, and a question asking you to classify one expects the cause to be named rather than the label alone.
Communicable, or infectious, diseases.
- Caused by pathogens — bacteria, viruses, protozoa, fungi or worms
- They spread from an infected person to a healthy one, either directly by contact or indirectly through air, water, food or a vector
- Examples: cholera, typhoid, tuberculosis, influenza, the common cold, measles, chicken pox, malaria, AIDS, ringworm, polio
Non-communicable, or non-infectious, diseases.
- Not caused by a pathogen, and they do not spread from person to person
- They arise from deficiency, heredity, degeneration, hormonal imbalance, allergy or the way a person lives
- Deficiency diseases: scurvy, rickets, anaemia, goitre, night blindness, kwashiorkor
- Hereditary diseases: haemophilia, colour blindness, sickle-cell anaemia
- Degenerative diseases: arthritis, cataract
- Hormonal disorders: diabetes, goitre
- Allergic conditions: asthma
- Others: cancer, high blood pressure, heart disease
Now the boundary case that catches people out. A non-communicable disease can still be extremely common in one area. Goitre is widespread wherever the soil and water are short of iodine, and whole villages may be affected.
That has nothing whatever to do with contagion. Everybody in the village has the same disease because everybody drinks the same iodine-poor water, not because they caught it from one another.
So "many people in one place have it" does not make a disease communicable. The test is whether it has a pathogen that passes from one person to another — and if it does not, then no amount of isolation, disinfection or quarantine will reduce it by a single case. What will reduce it is iodised salt, and that is why the distinction is practical rather than merely a matter of naming.
A second boundary worth knowing. Some conditions have more than one cause. Goitre is usually a deficiency disease and can also be hormonal; anaemia can come from iron deficiency, from blood loss, or from a hereditary defect. So a disease may appear in two of the lists, and a question asking you to classify one expects the cause to be named rather than the label alone.
What is the difference between endemic, epidemic, pandemic and sporadic?
These four words describe how widely and how regularly a disease occurs — not how serious it is.
Endemic — a disease constantly present in a particular area or population, occurring in small numbers all the time.
- It never goes away and never suddenly increases; it is simply always there
- Examples: malaria in certain parts of India; goitre in the sub-Himalayan belt where the soil lacks iodine
Epidemic — a disease that suddenly spreads to a large number of people in an area at one time, far above the level usually seen there.
- The key word is sudden and the key comparison is with the normal level for that place
- Examples: an outbreak of cholera after a flood has contaminated the water; dengue in a city after the monsoon
Pandemic — an epidemic that spreads across several countries or continents, affecting a very large population.
- The difference from an epidemic is purely one of geographical extent
- Examples: influenza spreading worldwide; AIDS
Sporadic — a disease that occurs occasionally, in a few scattered cases, with no regular pattern and no connection between them.
- Examples: occasional cases of tetanus; a stray case of cholera in an area with a good water supply
Now the point that gives this section its substance. All four terms are about distribution, and none of them says anything about severity.
- Tetanus is sporadic and it is one of the most dangerous diseases on the list
- The common cold can be epidemic in a school and does nobody any lasting harm
- Malaria is endemic in some districts and kills people every year in them
So a sporadic disease can be deadly and a pandemic one can be mild. These are epidemiological terms — they describe the pattern a disease makes across a population, which is what a public-health worker needs to know in order to respond.
And that is exactly why the terms are useful. An endemic disease needs a long, steady programme — iodised salt, drainage, bed nets. An epidemic needs an immediate emergency response — chlorinate the water, isolate the cases, find the source. The same disease can call for both, depending on which pattern it is showing, and cholera is the standard example: endemic in some areas, and capable of becoming epidemic in any of them after a flood.
A question asking you to distinguish the four wants the pattern named in each case — always present, sudden and local, sudden and international, or scattered and occasional — and an answer that reaches for "more serious" has misunderstood the whole set.
Endemic — a disease constantly present in a particular area or population, occurring in small numbers all the time.
- It never goes away and never suddenly increases; it is simply always there
- Examples: malaria in certain parts of India; goitre in the sub-Himalayan belt where the soil lacks iodine
Epidemic — a disease that suddenly spreads to a large number of people in an area at one time, far above the level usually seen there.
- The key word is sudden and the key comparison is with the normal level for that place
- Examples: an outbreak of cholera after a flood has contaminated the water; dengue in a city after the monsoon
Pandemic — an epidemic that spreads across several countries or continents, affecting a very large population.
- The difference from an epidemic is purely one of geographical extent
- Examples: influenza spreading worldwide; AIDS
Sporadic — a disease that occurs occasionally, in a few scattered cases, with no regular pattern and no connection between them.
- Examples: occasional cases of tetanus; a stray case of cholera in an area with a good water supply
Now the point that gives this section its substance. All four terms are about distribution, and none of them says anything about severity.
- Tetanus is sporadic and it is one of the most dangerous diseases on the list
- The common cold can be epidemic in a school and does nobody any lasting harm
- Malaria is endemic in some districts and kills people every year in them
So a sporadic disease can be deadly and a pandemic one can be mild. These are epidemiological terms — they describe the pattern a disease makes across a population, which is what a public-health worker needs to know in order to respond.
And that is exactly why the terms are useful. An endemic disease needs a long, steady programme — iodised salt, drainage, bed nets. An epidemic needs an immediate emergency response — chlorinate the water, isolate the cases, find the source. The same disease can call for both, depending on which pattern it is showing, and cholera is the standard example: endemic in some areas, and capable of becoming epidemic in any of them after a flood.
A question asking you to distinguish the four wants the pattern named in each case — always present, sudden and local, sudden and international, or scattered and occasional — and an answer that reaches for "more serious" has misunderstood the whole set.
How do diseases travel through air and through water?
Air-borne diseases are inhaled in droplets; water-borne diseases are swallowed in contaminated water or food.
Air-borne transmission, also called droplet infection.
- How it happens: an infected person coughs, sneezes, spits, laughs or talks and releases a spray of tiny droplets carrying the pathogen. A healthy person nearby inhales them. Dried droplets settled on dust can also be stirred up and breathed in
- Diseases: tuberculosis, the common cold, influenza, diphtheria, whooping cough, measles, chicken pox, mumps, pneumonia
- Prevention: cover the mouth and nose with a handkerchief while coughing or sneezing; do not spit in public places; avoid crowded, poorly ventilated rooms; keep rooms well ventilated and sunlit, since sunlight and fresh air destroy many of these pathogens; isolate the patient; disinfect the patient's articles; and immunise
Water-borne transmission.
- How it happens: the faeces or urine of an infected person contaminates a water source — through an open drain, a leaking latrine or defecation in the open. A healthy person then drinks that water, or eats food washed in it, or eats food handled by an infected person who has not washed their hands. Flies carry the pathogen from faeces directly onto food
- Diseases: cholera, typhoid, dysentery, diarrhoea, hepatitis (jaundice), polio, amoebiasis
- Prevention: drink boiled or properly chlorinated water; keep the drinking water source well away from drains and latrines; use sanitary latrines; wash hands with soap before eating and after using the toilet; wash fruit and vegetables; cover all food; treat sewage before discharge; and immunise where a vaccine exists
Two other routes belong in a complete answer. Contact — directly through the skin or through shared clothing and towels, as in ringworm, scabies and AIDS. And vector-borne, which the next section deals with.
Now the pattern in the water-borne list, and it is the most useful idea in this chapter. Look at where each of those pathogens lives and where it has to get to.
Every one of them lives in the intestine of an infected person, leaves in the faeces, and has to reach the mouth of the next person. That route has a name: the faecal-oral route.
So cholera, typhoid, dysentery, hepatitis A, polio and amoebiasis are all the same problem wearing different names — human waste reaching human mouths.
And that means one set of measures prevents the whole list at once. A sanitary latrine so the waste does not reach the water; soap so it does not reach the hands; boiling so anything that got into the water is killed; and covered food so flies cannot complete the journey.
That is why sanitation does more against these diseases than medicine does. A hospital treats the cases that occur; a latrine and a bar of soap prevent them from occurring. A question asking how water-borne disease is controlled should lead with the faecal-oral route, because once the route is named every measure on the list becomes an obvious consequence of it rather than an item to be remembered.
Air-borne transmission, also called droplet infection.
- How it happens: an infected person coughs, sneezes, spits, laughs or talks and releases a spray of tiny droplets carrying the pathogen. A healthy person nearby inhales them. Dried droplets settled on dust can also be stirred up and breathed in
- Diseases: tuberculosis, the common cold, influenza, diphtheria, whooping cough, measles, chicken pox, mumps, pneumonia
- Prevention: cover the mouth and nose with a handkerchief while coughing or sneezing; do not spit in public places; avoid crowded, poorly ventilated rooms; keep rooms well ventilated and sunlit, since sunlight and fresh air destroy many of these pathogens; isolate the patient; disinfect the patient's articles; and immunise
Water-borne transmission.
- How it happens: the faeces or urine of an infected person contaminates a water source — through an open drain, a leaking latrine or defecation in the open. A healthy person then drinks that water, or eats food washed in it, or eats food handled by an infected person who has not washed their hands. Flies carry the pathogen from faeces directly onto food
- Diseases: cholera, typhoid, dysentery, diarrhoea, hepatitis (jaundice), polio, amoebiasis
- Prevention: drink boiled or properly chlorinated water; keep the drinking water source well away from drains and latrines; use sanitary latrines; wash hands with soap before eating and after using the toilet; wash fruit and vegetables; cover all food; treat sewage before discharge; and immunise where a vaccine exists
Two other routes belong in a complete answer. Contact — directly through the skin or through shared clothing and towels, as in ringworm, scabies and AIDS. And vector-borne, which the next section deals with.
Now the pattern in the water-borne list, and it is the most useful idea in this chapter. Look at where each of those pathogens lives and where it has to get to.
Every one of them lives in the intestine of an infected person, leaves in the faeces, and has to reach the mouth of the next person. That route has a name: the faecal-oral route.
So cholera, typhoid, dysentery, hepatitis A, polio and amoebiasis are all the same problem wearing different names — human waste reaching human mouths.
And that means one set of measures prevents the whole list at once. A sanitary latrine so the waste does not reach the water; soap so it does not reach the hands; boiling so anything that got into the water is killed; and covered food so flies cannot complete the journey.
That is why sanitation does more against these diseases than medicine does. A hospital treats the cases that occur; a latrine and a bar of soap prevent them from occurring. A question asking how water-borne disease is controlled should lead with the faecal-oral route, because once the route is named every measure on the list becomes an obvious consequence of it rather than an item to be remembered.
How do the housefly, the mosquito and the cockroach spread disease, and how are they controlled?
A vector is an animal that carries a pathogen from an infected source to a healthy person, and the three in the syllabus do it in two quite different ways.
Housefly — a mechanical vector.
- How it spreads disease: it breeds and feeds in filth, garbage and faeces, and walks about on them. Pathogens stick to its hairy legs and body, and it also carries them in its gut and in the fluid it deposits while feeding. When it settles on exposed food it transfers them
- Diseases: cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, tuberculosis, conjunctivitis
- Control: proper disposal of garbage and faeces; covered dustbins; sanitary latrines; cover all food and drinking water; screen doors and windows; use fly traps and insecticides; clean and drain the damp rubbish in which the larvae breed
Mosquito — a biological vector.
- How it spreads disease: only the female bites, because she needs a meal of blood for her eggs. She takes the pathogen from an infected person's blood, the pathogen develops inside her, and she injects it into the next person she bites
- Diseases: the female Anopheles transmits malaria; the female Aedes transmits dengue, chikungunya and yellow fever; the female Culex transmits filariasis and encephalitis
- Control: remove stagnant water, because the larvae need still water — empty and scrub coolers, pots, tanks, tyres and flower pots every week; spray oil or kerosene on standing water so the larvae cannot reach the surface to breathe; introduce larvivorous fish such as Gambusia; prevent bites with mosquito nets, screens, repellents and full-sleeved clothing; spray insecticide; fill in ditches and drain marshy ground
Cockroach — a mechanical vector.
- How it spreads disease: it lives in drains, dark damp corners, kitchens and cupboards, feeds on waste and refuse, and carries pathogens on its body and in its droppings onto food, utensils and working surfaces
- Diseases: cholera, typhoid, dysentery, diarrhoea, food poisoning; and its droppings can trigger asthma and allergies
- Control: keep the kitchen clean and dry; store food in closed containers; block cracks, crevices and drain openings; remove waste daily; use baits and insecticides
Now the distinction that decides everything, and it is the point of this section.
- The housefly and the cockroach are mechanical vectors. The pathogen merely rides on the outside, and does not develop in the insect. The insect is a vehicle
- The mosquito is a biological vector. The pathogen multiplies and develops inside it, completing part of its life cycle there. The insect is a necessary host
That is why the control measures are not interchangeable.
Against a mechanical vector you can break the chain at the delivery end — cover the food, and the fly has nowhere to unload. The fly is still there and the disease is still prevented.
Against a biological vector you cannot, because the delivery is an injection into the blood and there is nothing to cover. So the mosquito itself has to be attacked — at the larval stage, in the stagnant water, before it ever flies — and that is why every measure against malaria is about water rather than about food.
A question asking why covering food prevents typhoid but not malaria is answered by exactly this, and a question asking why anti-malaria work concentrates on drains and coolers has the same answer read the other way round.
Housefly — a mechanical vector.
- How it spreads disease: it breeds and feeds in filth, garbage and faeces, and walks about on them. Pathogens stick to its hairy legs and body, and it also carries them in its gut and in the fluid it deposits while feeding. When it settles on exposed food it transfers them
- Diseases: cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, tuberculosis, conjunctivitis
- Control: proper disposal of garbage and faeces; covered dustbins; sanitary latrines; cover all food and drinking water; screen doors and windows; use fly traps and insecticides; clean and drain the damp rubbish in which the larvae breed
Mosquito — a biological vector.
- How it spreads disease: only the female bites, because she needs a meal of blood for her eggs. She takes the pathogen from an infected person's blood, the pathogen develops inside her, and she injects it into the next person she bites
- Diseases: the female Anopheles transmits malaria; the female Aedes transmits dengue, chikungunya and yellow fever; the female Culex transmits filariasis and encephalitis
- Control: remove stagnant water, because the larvae need still water — empty and scrub coolers, pots, tanks, tyres and flower pots every week; spray oil or kerosene on standing water so the larvae cannot reach the surface to breathe; introduce larvivorous fish such as Gambusia; prevent bites with mosquito nets, screens, repellents and full-sleeved clothing; spray insecticide; fill in ditches and drain marshy ground
Cockroach — a mechanical vector.
- How it spreads disease: it lives in drains, dark damp corners, kitchens and cupboards, feeds on waste and refuse, and carries pathogens on its body and in its droppings onto food, utensils and working surfaces
- Diseases: cholera, typhoid, dysentery, diarrhoea, food poisoning; and its droppings can trigger asthma and allergies
- Control: keep the kitchen clean and dry; store food in closed containers; block cracks, crevices and drain openings; remove waste daily; use baits and insecticides
Now the distinction that decides everything, and it is the point of this section.
- The housefly and the cockroach are mechanical vectors. The pathogen merely rides on the outside, and does not develop in the insect. The insect is a vehicle
- The mosquito is a biological vector. The pathogen multiplies and develops inside it, completing part of its life cycle there. The insect is a necessary host
That is why the control measures are not interchangeable.
Against a mechanical vector you can break the chain at the delivery end — cover the food, and the fly has nowhere to unload. The fly is still there and the disease is still prevented.
Against a biological vector you cannot, because the delivery is an injection into the blood and there is nothing to cover. So the mosquito itself has to be attacked — at the larval stage, in the stagnant water, before it ever flies — and that is why every measure against malaria is about water rather than about food.
A question asking why covering food prevents typhoid but not malaria is answered by exactly this, and a question asking why anti-malaria work concentrates on drains and coolers has the same answer read the other way round.
Exam tip
Exam tip: name the route, then the measure that breaks it
Define communicable by its cause AND its spread — caused by a pathogen and passing from an infected person to a healthy one. Non-communicable — no pathogen, and it does not spread.
Classify non-communicable diseases by cause: deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness), degenerative (arthritis, cataract), hormonal (diabetes), allergic (asthma), and cancer and heart disease.
A common disease is not necessarily a communicable one — goitre affects whole villages because the water lacks iodine.
The four distribution terms describe PATTERN, not severity. Endemic — always present in an area. Epidemic — a sudden large rise above the usual level locally. Pandemic — an epidemic across countries or continents. Sporadic — occasional scattered cases.
Give a named example for each of the four — malaria endemic, cholera epidemic after a flood, influenza pandemic, tetanus sporadic.
Air-borne is DROPLET infection — coughing, sneezing, spitting and talking, then inhaled. Name tuberculosis, influenza, the common cold, diphtheria, whooping cough, measles, chicken pox.
Air-borne prevention: cover the mouth, do not spit, avoid crowds, keep rooms ventilated and sunlit, isolate the patient, immunise.
Water-borne diseases follow the FAECAL-ORAL route — say the phrase. Name cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, amoebiasis.
Water-borne prevention: boiled water, sanitary latrines, soap, covered food, sewage treatment, water source away from drains.
Housefly and cockroach are MECHANICAL vectors; the mosquito is a BIOLOGICAL vector — the pathogen develops inside it. This distinction is worth a mark on its own.
Only the FEMALE mosquito bites, and name the genus with the disease — Anopheles malaria, Aedes dengue, Culex filariasis.
And for mosquito control lead with removing stagnant water, because that is where the larvae are — covering food does nothing at all.
Classify non-communicable diseases by cause: deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness), degenerative (arthritis, cataract), hormonal (diabetes), allergic (asthma), and cancer and heart disease.
A common disease is not necessarily a communicable one — goitre affects whole villages because the water lacks iodine.
The four distribution terms describe PATTERN, not severity. Endemic — always present in an area. Epidemic — a sudden large rise above the usual level locally. Pandemic — an epidemic across countries or continents. Sporadic — occasional scattered cases.
Give a named example for each of the four — malaria endemic, cholera epidemic after a flood, influenza pandemic, tetanus sporadic.
Air-borne is DROPLET infection — coughing, sneezing, spitting and talking, then inhaled. Name tuberculosis, influenza, the common cold, diphtheria, whooping cough, measles, chicken pox.
Air-borne prevention: cover the mouth, do not spit, avoid crowds, keep rooms ventilated and sunlit, isolate the patient, immunise.
Water-borne diseases follow the FAECAL-ORAL route — say the phrase. Name cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, amoebiasis.
Water-borne prevention: boiled water, sanitary latrines, soap, covered food, sewage treatment, water source away from drains.
Housefly and cockroach are MECHANICAL vectors; the mosquito is a BIOLOGICAL vector — the pathogen develops inside it. This distinction is worth a mark on its own.
Only the FEMALE mosquito bites, and name the genus with the disease — Anopheles malaria, Aedes dengue, Culex filariasis.
And for mosquito control lead with removing stagnant water, because that is where the larvae are — covering food does nothing at all.
Did you know
Why a bar of soap outperforms a hospital
Ask which single thing has done most to reduce death from infectious disease and the obvious answers are medicines and hospitals. The honest answer is less glamorous: clean water and somewhere safe to put human waste.
Look at the list of water-borne diseases again. Cholera, typhoid, dysentery, diarrhoea, hepatitis A, polio, amoebiasis. Every one of them lives in somebody's intestine, leaves in the faeces and has to reach another mouth.
That is a remarkably fragile arrangement. The pathogen has to survive outside a body, find a route to a mouth, and be swallowed in sufficient numbers. Break the route at any point and the whole journey fails.
So a latrine that keeps waste out of the water breaks it. A tap that delivers treated water breaks it. Washing hands with soap before eating breaks it. Covering the food so a fly cannot land on it breaks it. Boiling the water breaks it.
None of these involves a doctor, a drug or a diagnosis. And each one works against the entire list at once, because the list is really one problem with seven names.
Compare that with treatment. A medicine works against one pathogen, has to be given after somebody is already ill, has to reach them in time, and does nothing for the next person.
This is why public health has always been concerned with drains, wells, latrines and food stalls rather than with wards. And it is why the measures in this chapter can look disappointingly ordinary — cover the food, boil the water, wash your hands, dig a proper latrine — when they are in fact the most powerful interventions on the page.
A prevention that breaks a route prevents every disease using that route, and that is a return no treatment can match. Which is also why the exam answer to "how would you control water-borne disease in a village" should start with the latrine and the water supply, and not with the hospital.
Look at the list of water-borne diseases again. Cholera, typhoid, dysentery, diarrhoea, hepatitis A, polio, amoebiasis. Every one of them lives in somebody's intestine, leaves in the faeces and has to reach another mouth.
That is a remarkably fragile arrangement. The pathogen has to survive outside a body, find a route to a mouth, and be swallowed in sufficient numbers. Break the route at any point and the whole journey fails.
So a latrine that keeps waste out of the water breaks it. A tap that delivers treated water breaks it. Washing hands with soap before eating breaks it. Covering the food so a fly cannot land on it breaks it. Boiling the water breaks it.
None of these involves a doctor, a drug or a diagnosis. And each one works against the entire list at once, because the list is really one problem with seven names.
Compare that with treatment. A medicine works against one pathogen, has to be given after somebody is already ill, has to reach them in time, and does nothing for the next person.
This is why public health has always been concerned with drains, wells, latrines and food stalls rather than with wards. And it is why the measures in this chapter can look disappointingly ordinary — cover the food, boil the water, wash your hands, dig a proper latrine — when they are in fact the most powerful interventions on the page.
A prevention that breaks a route prevents every disease using that route, and that is a return no treatment can match. Which is also why the exam answer to "how would you control water-borne disease in a village" should start with the latrine and the water supply, and not with the hospital.
Exam relevance
Why does NEET keep returning to modes of transmission?
Because Human Health and Disease is a heavily examined Class 12 chapter, and the mode of transmission is asked for almost every disease in it.
This is the foundation for Class 12 Biology Human Health and Disease, examined in NEET. That chapter takes the same diseases and asks for four things about each: the causative organism, the mode of transmission, the symptoms and the prevention. Match-the-column questions pairing a disease with its pathogen or with its route are among the most predictable NEET items in the entire syllabus, and the routes on this page are the routes that chapter uses.
The vector distinction is examined by name. Class 12 names the female Anopheles for malaria, the female Aedes aegypti for dengue and chikungunya, and Culex for filariasis — the same three genera given here, with the same insistence that only the female bites. Questions specifying the genus and asking for the disease, or the reverse, are standard, and getting Aedes and Culex the right way round is exactly what they test.
The malarial life cycle makes the biological-vector idea quantitative. Class 12 traces Plasmodium through the mosquito and through the human host, with the sporozoites, the liver stage, the red blood cell stage and the gametocytes that the mosquito takes up. The point made here — that the parasite develops inside the mosquito — is why the mosquito is called the definitive host in that chapter, and it is the reason vector control is the primary strategy.
The faecal-oral route is examined as a group. Class 12 covers typhoid with Salmonella typhi, amoebiasis with Entamoeba histolytica, and ascariasis with Ascaris, all transmitted by contaminated water and food and by houseflies. The common route this page identifies is what lets you answer a whole set of questions from one idea, and the housefly appears in that chapter as the mechanical carrier.
Air-borne disease appears with the same examples. Pneumonia and the common cold are covered as droplet infections, with transmission by inhaled droplets and by contaminated articles — and tuberculosis and its BCG vaccination appear in the immunisation section.
The non-communicable list connects to several chapters. Deficiency diseases belong to the nutrition chapter; hereditary ones — haemophilia, colour blindness, sickle-cell anaemia and thalassaemia — are examined in Class 12 Principles of Inheritance and Variation as pedigree and inheritance problems; cancer has its own section in Human Health and Disease; and diabetes and goitre belong to Chemical Coordination and Integration. So the classification on this page is a map of where each disease is examined later.
The distribution terms appear in ecology and public health. Endemic is used in Class 12 Biodiversity and Conservation in a different but related sense — a species confined to one area — so the word is worth knowing in both senses.
What the questions look like. For board work, expect distinguish communicable from non-communicable disease with examples, define endemic, epidemic, pandemic and sporadic with an example each, describe air-borne and water-borne transmission with diseases and prevention, and explain the role of the housefly, mosquito and cockroach with control measures. Every answer wants named diseases. For NEET, expect disease-to-pathogen and disease-to-vector matching, the malarial life cycle, and the inheritance of the hereditary diseases.
How board and competitive emphasis differ. A board paper rewards the route named with its diseases and the measure that breaks it. A competitive paper assumes all three and asks for the causative organism's exact name, or the stage of a parasite in a named host.
The single trap that costs the most marks. Treating a mechanical and a biological vector as the same thing. In a mechanical vector the pathogen merely rides on the outside and does not develop — so covering the food defeats it. In a biological vector the pathogen completes part of its life cycle inside, and it is injected rather than deposited — so only destroying the insect or preventing the bite will work. The defence is to ask whether the pathogen grows inside the carrier, because that one question decides whether the control measure should protect the food or attack the breeding water.
This is the foundation for Class 12 Biology Human Health and Disease, examined in NEET. That chapter takes the same diseases and asks for four things about each: the causative organism, the mode of transmission, the symptoms and the prevention. Match-the-column questions pairing a disease with its pathogen or with its route are among the most predictable NEET items in the entire syllabus, and the routes on this page are the routes that chapter uses.
The vector distinction is examined by name. Class 12 names the female Anopheles for malaria, the female Aedes aegypti for dengue and chikungunya, and Culex for filariasis — the same three genera given here, with the same insistence that only the female bites. Questions specifying the genus and asking for the disease, or the reverse, are standard, and getting Aedes and Culex the right way round is exactly what they test.
The malarial life cycle makes the biological-vector idea quantitative. Class 12 traces Plasmodium through the mosquito and through the human host, with the sporozoites, the liver stage, the red blood cell stage and the gametocytes that the mosquito takes up. The point made here — that the parasite develops inside the mosquito — is why the mosquito is called the definitive host in that chapter, and it is the reason vector control is the primary strategy.
The faecal-oral route is examined as a group. Class 12 covers typhoid with Salmonella typhi, amoebiasis with Entamoeba histolytica, and ascariasis with Ascaris, all transmitted by contaminated water and food and by houseflies. The common route this page identifies is what lets you answer a whole set of questions from one idea, and the housefly appears in that chapter as the mechanical carrier.
Air-borne disease appears with the same examples. Pneumonia and the common cold are covered as droplet infections, with transmission by inhaled droplets and by contaminated articles — and tuberculosis and its BCG vaccination appear in the immunisation section.
The non-communicable list connects to several chapters. Deficiency diseases belong to the nutrition chapter; hereditary ones — haemophilia, colour blindness, sickle-cell anaemia and thalassaemia — are examined in Class 12 Principles of Inheritance and Variation as pedigree and inheritance problems; cancer has its own section in Human Health and Disease; and diabetes and goitre belong to Chemical Coordination and Integration. So the classification on this page is a map of where each disease is examined later.
The distribution terms appear in ecology and public health. Endemic is used in Class 12 Biodiversity and Conservation in a different but related sense — a species confined to one area — so the word is worth knowing in both senses.
What the questions look like. For board work, expect distinguish communicable from non-communicable disease with examples, define endemic, epidemic, pandemic and sporadic with an example each, describe air-borne and water-borne transmission with diseases and prevention, and explain the role of the housefly, mosquito and cockroach with control measures. Every answer wants named diseases. For NEET, expect disease-to-pathogen and disease-to-vector matching, the malarial life cycle, and the inheritance of the hereditary diseases.
How board and competitive emphasis differ. A board paper rewards the route named with its diseases and the measure that breaks it. A competitive paper assumes all three and asks for the causative organism's exact name, or the stage of a parasite in a named host.
The single trap that costs the most marks. Treating a mechanical and a biological vector as the same thing. In a mechanical vector the pathogen merely rides on the outside and does not develop — so covering the food defeats it. In a biological vector the pathogen completes part of its life cycle inside, and it is injected rather than deposited — so only destroying the insect or preventing the bite will work. The defence is to ask whether the pathogen grows inside the carrier, because that one question decides whether the control measure should protect the food or attack the breeding water.
Key takeaways
Categories of disease and how they spread: quick revision
- Communicable (infectious) — caused by a pathogen (bacteria, viruses, protozoa, fungi, worms) and spreads from an infected person to a healthy one. Cholera, typhoid, tuberculosis, influenza, measles, chicken pox, malaria, AIDS, ringworm.
- Non-communicable (non-infectious) — no pathogen, and it does not spread. Deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness, sickle-cell anaemia), degenerative (arthritis, cataract), hormonal (diabetes), allergic (asthma), plus cancer and heart disease.
- A widespread disease is not necessarily communicable — goitre affects whole villages because the water is short of iodine, and iodised salt is the remedy, not isolation.
- Endemic — constantly present in an area in small numbers. Malaria in parts of India; goitre in the sub-Himalayan belt.
- Epidemic — a sudden rise to a large number of cases in an area, far above the usual level. Cholera after a flood; dengue after the monsoon.
- Pandemic — an epidemic spread across several countries or continents. Influenza worldwide; AIDS.
- Sporadic — occasional scattered cases with no pattern. Tetanus.
- All four describe DISTRIBUTION, not severity — tetanus is sporadic and dangerous, a cold can be epidemic and harmless.
- Air-borne (droplet infection) — released by coughing, sneezing, spitting or talking and then inhaled. Tuberculosis, common cold, influenza, diphtheria, whooping cough, measles, chicken pox, mumps, pneumonia.
- Air-borne prevention — cover the mouth and nose, do not spit, avoid crowded ill-ventilated places, keep rooms ventilated and sunlit, isolate the patient, disinfect articles, immunise.
- Water-borne — the pathogen leaves in the faeces, contaminates water or food, and is swallowed. Cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, amoebiasis.
- Water-borne prevention — boiled or chlorinated water, sanitary latrines, soap before eating and after the toilet, wash fruit and vegetables, cover food, treat sewage, keep the water source away from drains.
- Every water-borne disease on the list uses the FAECAL-ORAL route, so one set of measures prevents all of them — which is why sanitation outperforms treatment.
- Housefly — a MECHANICAL vector. Breeds in filth; germs stick to its hairy legs and body. Cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, tuberculosis, conjunctivitis. Control: dispose of garbage and faeces, covered bins, cover food, screens, traps, insecticides.
- Mosquito — a BIOLOGICAL vector. Only the female bites; the pathogen develops inside her. Anopheles malaria; Aedes dengue and chikungunya; Culex filariasis. Control: remove stagnant water, oil standing water, larvivorous fish (Gambusia), nets, screens, repellents, full sleeves, insecticide, fill ditches.
- Cockroach — a MECHANICAL vector. Lives in drains and dark damp corners; carries germs on its body and droppings. Cholera, typhoid, dysentery, food poisoning, and it triggers asthma. Control: clean dry kitchen, closed containers, block cracks and drains, remove waste daily.
- The distinction decides the control: a mechanical vector is defeated by covering the food, a biological one only by destroying the insect or preventing the bite — which is why anti-malaria work is about water and not about food.
Name any disease on this page, state its route, and then name the one measure that breaks that route — if all three come together, this chapter is finished.
- Non-communicable (non-infectious) — no pathogen, and it does not spread. Deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness, sickle-cell anaemia), degenerative (arthritis, cataract), hormonal (diabetes), allergic (asthma), plus cancer and heart disease.
- A widespread disease is not necessarily communicable — goitre affects whole villages because the water is short of iodine, and iodised salt is the remedy, not isolation.
- Endemic — constantly present in an area in small numbers. Malaria in parts of India; goitre in the sub-Himalayan belt.
- Epidemic — a sudden rise to a large number of cases in an area, far above the usual level. Cholera after a flood; dengue after the monsoon.
- Pandemic — an epidemic spread across several countries or continents. Influenza worldwide; AIDS.
- Sporadic — occasional scattered cases with no pattern. Tetanus.
- All four describe DISTRIBUTION, not severity — tetanus is sporadic and dangerous, a cold can be epidemic and harmless.
- Air-borne (droplet infection) — released by coughing, sneezing, spitting or talking and then inhaled. Tuberculosis, common cold, influenza, diphtheria, whooping cough, measles, chicken pox, mumps, pneumonia.
- Air-borne prevention — cover the mouth and nose, do not spit, avoid crowded ill-ventilated places, keep rooms ventilated and sunlit, isolate the patient, disinfect articles, immunise.
- Water-borne — the pathogen leaves in the faeces, contaminates water or food, and is swallowed. Cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, amoebiasis.
- Water-borne prevention — boiled or chlorinated water, sanitary latrines, soap before eating and after the toilet, wash fruit and vegetables, cover food, treat sewage, keep the water source away from drains.
- Every water-borne disease on the list uses the FAECAL-ORAL route, so one set of measures prevents all of them — which is why sanitation outperforms treatment.
- Housefly — a MECHANICAL vector. Breeds in filth; germs stick to its hairy legs and body. Cholera, typhoid, dysentery, diarrhoea, hepatitis, polio, tuberculosis, conjunctivitis. Control: dispose of garbage and faeces, covered bins, cover food, screens, traps, insecticides.
- Mosquito — a BIOLOGICAL vector. Only the female bites; the pathogen develops inside her. Anopheles malaria; Aedes dengue and chikungunya; Culex filariasis. Control: remove stagnant water, oil standing water, larvivorous fish (Gambusia), nets, screens, repellents, full sleeves, insecticide, fill ditches.
- Cockroach — a MECHANICAL vector. Lives in drains and dark damp corners; carries germs on its body and droppings. Cholera, typhoid, dysentery, food poisoning, and it triggers asthma. Control: clean dry kitchen, closed containers, block cracks and drains, remove waste daily.
- The distinction decides the control: a mechanical vector is defeated by covering the food, a biological one only by destroying the insect or preventing the bite — which is why anti-malaria work is about water and not about food.
Name any disease on this page, state its route, and then name the one measure that breaks that route — if all three come together, this chapter is finished.