Four Different Kinds of Germ Cause the Diseases You Know Best
Learn what a disease is and which ones spread, match each named disease to the kind of germ that causes it, follow the five routes by which infection travels, and see how each route is blocked.
Why does one medicine never work on every infection?
Because the diseases are caused by completely different kinds of organism.
Tuberculosis is caused by a bacterium. Malaria by a protozoan. Ringworm by a fungus. AIDS by a virus. These four are as unlike one another as a cow is unlike a mushroom, so a substance that kills one may have no effect at all on another.
That is why an antibiotic clears a bacterial infection and does nothing whatever for a cold, and why identifying the kind of germ is the first step in treating anything. This page covers the first part of the ICSE Class 8 Biology chapter on diseases: what a disease is, what causes them, how they travel, and how each route is blocked.
Tuberculosis is caused by a bacterium. Malaria by a protozoan. Ringworm by a fungus. AIDS by a virus. These four are as unlike one another as a cow is unlike a mushroom, so a substance that kills one may have no effect at all on another.
That is why an antibiotic clears a bacterial infection and does nothing whatever for a cold, and why identifying the kind of germ is the first step in treating anything. This page covers the first part of the ICSE Class 8 Biology chapter on diseases: what a disease is, what causes them, how they travel, and how each route is blocked.
Which diseases spread and which do not?
A disease is any condition that disturbs the normal functioning of the body or of one of its organs. The signs by which it is recognised are its symptoms.
Communicable diseases, also called infectious diseases, are caused by pathogens and spread from an infected person to a healthy one.
Examples: the common cold, influenza, tuberculosis, typhoid, cholera, malaria, dengue, measles, chickenpox, ringworm, hepatitis and AIDS.
Non-communicable diseases, also called non-infectious, do not spread from person to person. Sitting beside someone who has one carries no risk at all. They arise from causes within the body or from the way a person lives:
- Deficiency diseases, from a shortage of a nutrient — scurvy (vitamin C), rickets (vitamin D), anaemia (iron), goitre (iodine), night blindness (vitamin A), beriberi (vitamin B)
- Hereditary diseases, passed from parents through the genes — haemophilia, colour blindness, thalassaemia
- Organ malfunction — diabetes, from too little insulin
- Degenerative diseases, from the wearing out of tissues with age — arthritis
- Allergies — asthma
- Lifestyle diseases — heart disease and high blood pressure
- Cancer
A pathogen is any organism that causes disease. The word germ is the everyday term for the same thing.
The distinction that matters practically. A communicable disease can be prevented by breaking the route it travels — clean water, covered food, mosquito nets, vaccination. A non-communicable disease has no route to break, so it is prevented in a completely different way: by correcting the diet, or by changing a habit, or in the case of a hereditary disease, not at all.
A misconception worth clearing. Non-communicable does not mean mild. Heart disease, cancer and diabetes are all non-communicable and all serious. The word describes only whether the disease can pass to another person, and nothing about how dangerous it is.
Communicable diseases, also called infectious diseases, are caused by pathogens and spread from an infected person to a healthy one.
Examples: the common cold, influenza, tuberculosis, typhoid, cholera, malaria, dengue, measles, chickenpox, ringworm, hepatitis and AIDS.
Non-communicable diseases, also called non-infectious, do not spread from person to person. Sitting beside someone who has one carries no risk at all. They arise from causes within the body or from the way a person lives:
- Deficiency diseases, from a shortage of a nutrient — scurvy (vitamin C), rickets (vitamin D), anaemia (iron), goitre (iodine), night blindness (vitamin A), beriberi (vitamin B)
- Hereditary diseases, passed from parents through the genes — haemophilia, colour blindness, thalassaemia
- Organ malfunction — diabetes, from too little insulin
- Degenerative diseases, from the wearing out of tissues with age — arthritis
- Allergies — asthma
- Lifestyle diseases — heart disease and high blood pressure
- Cancer
A pathogen is any organism that causes disease. The word germ is the everyday term for the same thing.
The distinction that matters practically. A communicable disease can be prevented by breaking the route it travels — clean water, covered food, mosquito nets, vaccination. A non-communicable disease has no route to break, so it is prevented in a completely different way: by correcting the diet, or by changing a habit, or in the case of a hereditary disease, not at all.
A misconception worth clearing. Non-communicable does not mean mild. Heart disease, cancer and diabetes are all non-communicable and all serious. The word describes only whether the disease can pass to another person, and nothing about how dangerous it is.
Which pathogen causes which disease?
Pathogens fall into five groups, and each group causes a recognisable set of diseases.
Bacteria — single-celled organisms:
- Tuberculosis, caused by Mycobacterium tuberculosis, which chiefly attacks the lungs
- Typhoid, caused by Salmonella typhi
- Cholera, caused by Vibrio cholerae
- Also tetanus, diphtheria, whooping cough, pneumonia and leprosy
Viruses — far smaller than bacteria, and able to multiply only inside a living cell:
- AIDS, caused by HIV — the human immunodeficiency virus
- Common cold, influenza, measles, mumps, chickenpox, polio, rabies, dengue and hepatitis
Protozoa — single-celled animals:
- Malaria, caused by Plasmodium
- Amoebic dysentery, caused by Entamoeba
- Kala-azar and sleeping sickness
Fungi:
- Ringworm, and the related athlete's foot and dhobi itch — all skin infections showing as itchy rings or patches
Worms, also called helminths:
- Tapeworm and roundworm in the intestine
- Filariasis, or elephantiasis, caused by a worm carried by a mosquito
The four the syllabus names, kept together:
- Tuberculosis — bacterium
- Malaria — protozoan
- Ringworm — fungus
- AIDS — virus
Why the kind of pathogen decides the treatment. Antibiotics act on bacteria and are useless against a virus, because a virus is not a living cell in the ordinary sense and has none of the machinery an antibiotic attacks. A fungal skin infection needs an antifungal cream; malaria needs an antimalarial drug.
This is the reason taking an antibiotic for a cold does no good — and why doing it repeatedly does active harm, by giving the body's harmless bacteria practice at surviving the drug.
A boundary case. A mosquito is not a pathogen. It is a vector — a carrier that transports the pathogen from one person to another. The pathogen of malaria is Plasmodium, and the mosquito merely delivers it, which is the subject of the next section.
Bacteria — single-celled organisms:
- Tuberculosis, caused by Mycobacterium tuberculosis, which chiefly attacks the lungs
- Typhoid, caused by Salmonella typhi
- Cholera, caused by Vibrio cholerae
- Also tetanus, diphtheria, whooping cough, pneumonia and leprosy
Viruses — far smaller than bacteria, and able to multiply only inside a living cell:
- AIDS, caused by HIV — the human immunodeficiency virus
- Common cold, influenza, measles, mumps, chickenpox, polio, rabies, dengue and hepatitis
Protozoa — single-celled animals:
- Malaria, caused by Plasmodium
- Amoebic dysentery, caused by Entamoeba
- Kala-azar and sleeping sickness
Fungi:
- Ringworm, and the related athlete's foot and dhobi itch — all skin infections showing as itchy rings or patches
Worms, also called helminths:
- Tapeworm and roundworm in the intestine
- Filariasis, or elephantiasis, caused by a worm carried by a mosquito
The four the syllabus names, kept together:
- Tuberculosis — bacterium
- Malaria — protozoan
- Ringworm — fungus
- AIDS — virus
Why the kind of pathogen decides the treatment. Antibiotics act on bacteria and are useless against a virus, because a virus is not a living cell in the ordinary sense and has none of the machinery an antibiotic attacks. A fungal skin infection needs an antifungal cream; malaria needs an antimalarial drug.
This is the reason taking an antibiotic for a cold does no good — and why doing it repeatedly does active harm, by giving the body's harmless bacteria practice at surviving the drug.
A boundary case. A mosquito is not a pathogen. It is a vector — a carrier that transports the pathogen from one person to another. The pathogen of malaria is Plasmodium, and the mosquito merely delivers it, which is the subject of the next section.
How do communicable diseases travel from person to person?
By five routes, and every communicable disease uses one or more of them.
Through air — droplet infection. Coughing, sneezing, talking and spitting release tiny droplets carrying the pathogen, which another person breathes in.
Examples: common cold, influenza, tuberculosis, measles, chickenpox, whooping cough, pneumonia.
This is why a person with a cold should cover the mouth and nose with a handkerchief, and why crowded, poorly ventilated rooms spread infection so effectively.
Through water. Drinking water contaminated with sewage carries pathogens straight into the gut.
Examples: cholera, typhoid, dysentery, hepatitis and jaundice, polio.
Through food. Stale or contaminated food, unwashed fruit and vegetables, and food on which houseflies have settled.
Examples: food poisoning, cholera, typhoid, dysentery, and worm infections.
Through contact. Direct touch with an infected person, or use of their belongings — towels, combs, clothes.
Examples: ringworm, scabies, chickenpox, conjunctivitis. AIDS spreads through blood and body fluids, including through shared needles and unscreened blood transfusion.
Through vectors. A vector is an animal, usually an insect, that carries the pathogen from one person to another.
- **Female Anopheles mosquito — malaria
- Female Aedes mosquito — dengue and chikungunya
- Culex mosquito — filariasis
- Housefly — cholera, typhoid and dysentery, carried on its feet and body from filth to food
- Rat flea — plague
- Sandfly — kala-azar
Notice which mosquito does what. Only the female** mosquito bites, since she needs blood to develop her eggs, and different genera carry different diseases. Writing mosquito without naming the genus is usually marked as incomplete.
Why knowing the route is the whole point. Each route has its own block. An air-borne disease is checked by covering the mouth and by ventilation; a water-borne one by boiling or filtering water; a food-borne one by covering food and washing hands; a contact disease by not sharing personal articles; and a vector-borne one by destroying the vector. That is the subject of the last section, and it follows entirely from this one.
Through air — droplet infection. Coughing, sneezing, talking and spitting release tiny droplets carrying the pathogen, which another person breathes in.
Examples: common cold, influenza, tuberculosis, measles, chickenpox, whooping cough, pneumonia.
This is why a person with a cold should cover the mouth and nose with a handkerchief, and why crowded, poorly ventilated rooms spread infection so effectively.
Through water. Drinking water contaminated with sewage carries pathogens straight into the gut.
Examples: cholera, typhoid, dysentery, hepatitis and jaundice, polio.
Through food. Stale or contaminated food, unwashed fruit and vegetables, and food on which houseflies have settled.
Examples: food poisoning, cholera, typhoid, dysentery, and worm infections.
Through contact. Direct touch with an infected person, or use of their belongings — towels, combs, clothes.
Examples: ringworm, scabies, chickenpox, conjunctivitis. AIDS spreads through blood and body fluids, including through shared needles and unscreened blood transfusion.
Through vectors. A vector is an animal, usually an insect, that carries the pathogen from one person to another.
- **Female Anopheles mosquito — malaria
- Female Aedes mosquito — dengue and chikungunya
- Culex mosquito — filariasis
- Housefly — cholera, typhoid and dysentery, carried on its feet and body from filth to food
- Rat flea — plague
- Sandfly — kala-azar
Notice which mosquito does what. Only the female** mosquito bites, since she needs blood to develop her eggs, and different genera carry different diseases. Writing mosquito without naming the genus is usually marked as incomplete.
Why knowing the route is the whole point. Each route has its own block. An air-borne disease is checked by covering the mouth and by ventilation; a water-borne one by boiling or filtering water; a food-borne one by covering food and washing hands; a contact disease by not sharing personal articles; and a vector-borne one by destroying the vector. That is the subject of the last section, and it follows entirely from this one.
How are diseases prevented and controlled?
By breaking the route the pathogen travels, and by preparing the body in advance.
Personal hygiene — blocking contact and food routes:
- Bathe daily and wear clean clothes
- Wash hands with soap before eating and after using the toilet, which is the single most effective habit in this list
- Trim the nails short, since dirt and germs lodge under them
- Cover the mouth and nose when coughing or sneezing, using a handkerchief
- Do not share towels, combs, handkerchiefs or utensils
- Brush the teeth twice daily
Sanitation and public hygiene — blocking the water and food routes at their source:
- Proper disposal of waste and sewage, in covered drains and away from water supplies
- A clean drinking-water supply, treated and piped
- Covered dustbins, and no rubbish heaps near homes
- Keeping surroundings clean, since filth is where flies breed
Safe food and water:
- Boil or filter drinking water, especially during the rains
- Wash fruits and vegetables before eating
- Cover food to keep flies off, and eat freshly cooked food
Vector control — attacking the carrier rather than the pathogen:
- Remove stagnant water from coolers, flowerpots, tyres, tins and open tanks, since mosquitoes breed in still water
- Fill up ditches and drain marshy ground
- Spray insecticide, and spread a thin film of oil on standing water to stop larvae breathing
- Introduce larvivorous fish into ponds, which eat mosquito larvae
- Use mosquito nets, screens, repellents and full-sleeved clothing
- Control rats and flies
Immunisation — preparing the body before it ever meets the pathogen. A vaccine contains the pathogen in a dead or weakened form, harmless in itself but enough for the body to make antibodies against it. If the real pathogen arrives later, the antibodies are ready.
Common vaccines: BCG for tuberculosis, DPT for diphtheria, whooping cough and tetanus, polio drops, and vaccines for measles and hepatitis B.
Two more measures, used when someone is already ill: isolation of the patient so the disease does not spread, and disinfection of their room and belongings.
The distinction to keep clear. A vaccine prevents a disease and is given to a healthy person before exposure. A medicine treats a disease and is given to a sick person afterwards. They are not alternatives to each other, and vaccination is by far the cheaper of the two — which is why it is done on a national scale for children.
Personal hygiene — blocking contact and food routes:
- Bathe daily and wear clean clothes
- Wash hands with soap before eating and after using the toilet, which is the single most effective habit in this list
- Trim the nails short, since dirt and germs lodge under them
- Cover the mouth and nose when coughing or sneezing, using a handkerchief
- Do not share towels, combs, handkerchiefs or utensils
- Brush the teeth twice daily
Sanitation and public hygiene — blocking the water and food routes at their source:
- Proper disposal of waste and sewage, in covered drains and away from water supplies
- A clean drinking-water supply, treated and piped
- Covered dustbins, and no rubbish heaps near homes
- Keeping surroundings clean, since filth is where flies breed
Safe food and water:
- Boil or filter drinking water, especially during the rains
- Wash fruits and vegetables before eating
- Cover food to keep flies off, and eat freshly cooked food
Vector control — attacking the carrier rather than the pathogen:
- Remove stagnant water from coolers, flowerpots, tyres, tins and open tanks, since mosquitoes breed in still water
- Fill up ditches and drain marshy ground
- Spray insecticide, and spread a thin film of oil on standing water to stop larvae breathing
- Introduce larvivorous fish into ponds, which eat mosquito larvae
- Use mosquito nets, screens, repellents and full-sleeved clothing
- Control rats and flies
Immunisation — preparing the body before it ever meets the pathogen. A vaccine contains the pathogen in a dead or weakened form, harmless in itself but enough for the body to make antibodies against it. If the real pathogen arrives later, the antibodies are ready.
Common vaccines: BCG for tuberculosis, DPT for diphtheria, whooping cough and tetanus, polio drops, and vaccines for measles and hepatitis B.
Two more measures, used when someone is already ill: isolation of the patient so the disease does not spread, and disinfection of their room and belongings.
The distinction to keep clear. A vaccine prevents a disease and is given to a healthy person before exposure. A medicine treats a disease and is given to a sick person afterwards. They are not alternatives to each other, and vaccination is by far the cheaper of the two — which is why it is done on a national scale for children.
Exam tip
Exam tip: name the pathogen, not just the kind
Where the syllabus gives a name, use it: tuberculosis by Mycobacterium tuberculosis, malaria by Plasmodium, AIDS by HIV, cholera by Vibrio cholerae, typhoid by Salmonella typhi.
State the kind of pathogen as well — bacterium, virus, protozoan, fungus or worm. Questions often ask for the kind and the name separately.
Never call a mosquito a pathogen. It is a vector, and the pathogen is what it carries.
Name the genus of mosquito: Anopheles for malaria, Aedes for dengue, Culex for filariasis. And say female, since only she bites.
When giving a mode of transmission, pair it with a disease: water-borne, such as cholera and typhoid.
For a prevention question, match the measure to the route. Boiling water blocks the water route; a mosquito net blocks the vector route; washing hands blocks the contact and food routes. An unsorted list of good habits earns less.
Say mosquitoes breed in stagnant water, and give at least three places it collects — coolers, flowerpots, discarded tyres.
Explain a vaccine as a dead or weakened pathogen that makes the body produce antibodies in advance, and distinguish it from a medicine, which treats after the illness has begun.
And remember antibiotics do not work on viruses — a favourite one-mark question.
State the kind of pathogen as well — bacterium, virus, protozoan, fungus or worm. Questions often ask for the kind and the name separately.
Never call a mosquito a pathogen. It is a vector, and the pathogen is what it carries.
Name the genus of mosquito: Anopheles for malaria, Aedes for dengue, Culex for filariasis. And say female, since only she bites.
When giving a mode of transmission, pair it with a disease: water-borne, such as cholera and typhoid.
For a prevention question, match the measure to the route. Boiling water blocks the water route; a mosquito net blocks the vector route; washing hands blocks the contact and food routes. An unsorted list of good habits earns less.
Say mosquitoes breed in stagnant water, and give at least three places it collects — coolers, flowerpots, discarded tyres.
Explain a vaccine as a dead or weakened pathogen that makes the body produce antibodies in advance, and distinguish it from a medicine, which treats after the illness has begun.
And remember antibiotics do not work on viruses — a favourite one-mark question.
Did you know
Why does draining a flowerpot do more against dengue than any medicine?
There is no medicine that kills the dengue virus directly. Treatment manages the fever and the fluid loss while the body's own defences do the work.
But the virus cannot reach a person at all without an Aedes mosquito to carry it, and that mosquito has a weakness: it breeds only in clean, still water, in small collections close to houses. The saucer under a flowerpot, the tray behind a refrigerator, the water in a desert cooler, a discarded tyre, an uncovered tank.
Empty those and the mosquitoes have nowhere to complete their life cycle. The virus may still exist in the neighbourhood, but it has lost its transport.
This is why a municipal team inspecting flowerpots is doing serious medicine, and it is the clearest illustration of the principle behind the whole chapter: a communicable disease needs a route, and blocking the route is usually easier, cheaper and more certain than attacking the pathogen once it has arrived.
But the virus cannot reach a person at all without an Aedes mosquito to carry it, and that mosquito has a weakness: it breeds only in clean, still water, in small collections close to houses. The saucer under a flowerpot, the tray behind a refrigerator, the water in a desert cooler, a discarded tyre, an uncovered tank.
Empty those and the mosquitoes have nowhere to complete their life cycle. The virus may still exist in the neighbourhood, but it has lost its transport.
This is why a municipal team inspecting flowerpots is doing serious medicine, and it is the clearest illustration of the principle behind the whole chapter: a communicable disease needs a route, and blocking the route is usually easier, cheaper and more certain than attacking the pathogen once it has arrived.
Key takeaways
Diseases, pathogens and transmission: quick revision
- A disease disturbs the normal functioning of the body; its signs are symptoms.
- Communicable diseases are caused by pathogens and spread between people — cold, influenza, tuberculosis, typhoid, cholera, malaria, dengue, measles, chickenpox, ringworm, hepatitis, AIDS.
- Non-communicable diseases do not spread: deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness, thalassaemia), organ malfunction (diabetes), degenerative (arthritis), allergy (asthma), lifestyle and cancer. Non-communicable does not mean mild.
- Pathogen groups — bacteria (tuberculosis by Mycobacterium tuberculosis, typhoid, cholera, tetanus, leprosy), viruses (AIDS by HIV, cold, influenza, measles, polio, dengue, hepatitis), protozoa (malaria by Plasmodium, amoebic dysentery), fungi (ringworm, athlete's foot), worms (tapeworm, roundworm, filariasis).
- The four named: tuberculosis — bacterium, malaria — protozoan, ringworm — fungus, AIDS — virus.
- Antibiotics act on bacteria and not on viruses.
- Modes of transmission — air (droplets: cold, influenza, tuberculosis, measles), water (cholera, typhoid, dysentery, hepatitis), food (food poisoning, worms, flies on food), contact (ringworm, scabies, conjunctivitis; AIDS by blood and body fluids), vectors.
- Vectors: female Anopheles — malaria; female Aedes — dengue; Culex — filariasis; housefly — cholera, typhoid, dysentery; rat flea — plague; sandfly — kala-azar. A mosquito is a vector, not a pathogen.
- Prevention works by breaking the route: personal hygiene (washing hands with soap, covering the mouth, not sharing articles), sanitation (waste and sewage disposal, clean water supply), safe food and water (boiling, washing, covering), vector control (removing stagnant water, insecticide, larvivorous fish, nets and full sleeves).
- Immunisation gives a dead or weakened pathogen so the body makes antibodies in advance — BCG, DPT, polio, measles, hepatitis B. A vaccine prevents; a medicine treats.
- Isolation and disinfection are used once someone is ill.
Test yourself by naming the pathogen and its kind for a dozen diseases, then matching each mode of transmission to the measure that blocks it — those two tables carry most of the marks here.
- Communicable diseases are caused by pathogens and spread between people — cold, influenza, tuberculosis, typhoid, cholera, malaria, dengue, measles, chickenpox, ringworm, hepatitis, AIDS.
- Non-communicable diseases do not spread: deficiency (scurvy, rickets, anaemia, goitre), hereditary (haemophilia, colour blindness, thalassaemia), organ malfunction (diabetes), degenerative (arthritis), allergy (asthma), lifestyle and cancer. Non-communicable does not mean mild.
- Pathogen groups — bacteria (tuberculosis by Mycobacterium tuberculosis, typhoid, cholera, tetanus, leprosy), viruses (AIDS by HIV, cold, influenza, measles, polio, dengue, hepatitis), protozoa (malaria by Plasmodium, amoebic dysentery), fungi (ringworm, athlete's foot), worms (tapeworm, roundworm, filariasis).
- The four named: tuberculosis — bacterium, malaria — protozoan, ringworm — fungus, AIDS — virus.
- Antibiotics act on bacteria and not on viruses.
- Modes of transmission — air (droplets: cold, influenza, tuberculosis, measles), water (cholera, typhoid, dysentery, hepatitis), food (food poisoning, worms, flies on food), contact (ringworm, scabies, conjunctivitis; AIDS by blood and body fluids), vectors.
- Vectors: female Anopheles — malaria; female Aedes — dengue; Culex — filariasis; housefly — cholera, typhoid, dysentery; rat flea — plague; sandfly — kala-azar. A mosquito is a vector, not a pathogen.
- Prevention works by breaking the route: personal hygiene (washing hands with soap, covering the mouth, not sharing articles), sanitation (waste and sewage disposal, clean water supply), safe food and water (boiling, washing, covering), vector control (removing stagnant water, insecticide, larvivorous fish, nets and full sleeves).
- Immunisation gives a dead or weakened pathogen so the body makes antibodies in advance — BCG, DPT, polio, measles, hepatitis B. A vaccine prevents; a medicine treats.
- Isolation and disinfection are used once someone is ill.
Test yourself by naming the pathogen and its kind for a dozen diseases, then matching each mode of transmission to the measure that blocks it — those two tables carry most of the marks here.