How a Mosquito Bite Carries Malaria From One Person to the Next
Learn the causes, spread, symptoms and prevention of common viral, bacterial and protozoal diseases, the life cycle of Plasmodium, ascariasis and filariasis, and the types, features, diagnosis and treatment of cancer.
Why do some diseases spread from person to person?
Health means physical, mental and social well-being, not just the absence of disease. Some diseases, caused by viruses, bacteria, protozoa or worms, pass between people through air, water, food or insects, while others, such as cancer, arise within the body itself.
This lesson covers common infectious diseases, malaria and worm infections, and cancer.
This lesson covers common infectious diseases, malaria and worm infections, and cancer.
What causes common viral, bacterial and protozoal diseases, and how can they be prevented?
Infectious diseases are caused by pathogens — typhoid and pneumonia by bacteria, the common cold by viruses and amoebiasis by a protozoan — and they spread through contaminated food and water, airborne droplets or close contact, so hygiene and vaccination prevent many of them.
Typhoid (bacterial):
- Caused by Salmonella typhi, spread through contaminated food and water
- Sustained high fever, weakness, stomach pain, constipation, headache and loss of appetite
- Confirmed by the Widal test
Pneumonia (bacterial):
- Caused by Streptococcus pneumoniae and Haemophilus influenzae, which infect the alveoli so that they fill with fluid
- Spreads through droplets and shared glasses or utensils
- Fever, chills, cough and, in severe cases, bluish lips and fingernails
Common cold (viral):
- Caused by rhinoviruses, which infect the nose and respiratory passage but not the lungs
- Nasal congestion, sore throat, cough and tiredness
- Spreads through droplets from coughs and sneezes and through contaminated objects
Amoebiasis (protozoal):
- Caused by Entamoeba histolytica in the large intestine
- Houseflies carry the parasite from faeces to food
- Abdominal pain, cramps and stools with mucus and blood
Prevention. Safe drinking water, proper sewage disposal, personal hygiene, control of flies and mosquitoes, and vaccination.
An everyday example. Uncovered chutneys at a roadside stall can spread typhoid and amoebiasis when flies or dirty water contaminate them.
The substance. Antibiotics can cure typhoid and pneumonia but not the common cold — a viral infection does not respond to antibiotics.
Typhoid (bacterial):
- Caused by Salmonella typhi, spread through contaminated food and water
- Sustained high fever, weakness, stomach pain, constipation, headache and loss of appetite
- Confirmed by the Widal test
Pneumonia (bacterial):
- Caused by Streptococcus pneumoniae and Haemophilus influenzae, which infect the alveoli so that they fill with fluid
- Spreads through droplets and shared glasses or utensils
- Fever, chills, cough and, in severe cases, bluish lips and fingernails
Common cold (viral):
- Caused by rhinoviruses, which infect the nose and respiratory passage but not the lungs
- Nasal congestion, sore throat, cough and tiredness
- Spreads through droplets from coughs and sneezes and through contaminated objects
Amoebiasis (protozoal):
- Caused by Entamoeba histolytica in the large intestine
- Houseflies carry the parasite from faeces to food
- Abdominal pain, cramps and stools with mucus and blood
Prevention. Safe drinking water, proper sewage disposal, personal hygiene, control of flies and mosquitoes, and vaccination.
An everyday example. Uncovered chutneys at a roadside stall can spread typhoid and amoebiasis when flies or dirty water contaminate them.
The substance. Antibiotics can cure typhoid and pneumonia but not the common cold — a viral infection does not respond to antibiotics.
What is the life cycle of Plasmodium, and what are ascariasis and filariasis?
Plasmodium, the malaria parasite, completes its life cycle in two hosts — an asexual phase in human liver and red blood cells and a sexual phase in the female Anopheles mosquito — while ascariasis and filariasis are worm diseases spread through contaminated soil and mosquito bites.
Life cycle of Plasmodium:
- An infected female Anopheles mosquito injects sporozoites while biting
- The sporozoites multiply asexually in liver cells, which burst and release parasites into the blood
- The parasites multiply inside red blood cells and burst them, releasing a toxin called haemozoin, which causes recurring chills and high fever
- Some parasites develop into sexual stages, gametocytes, in the blood
- A mosquito biting the person takes up gametocytes, and fertilisation and development follow in the mosquito's stomach wall
- New sporozoites move to the mosquito's salivary glands, ready to infect another person
Ascariasis:
- Caused by the roundworm Ascaris, which lives in the intestine
- Eggs passed in faeces contaminate soil, water and vegetables
- Internal bleeding, muscular pain, fever, anaemia and blockage of the intestine
Filariasis (elephantiasis):
- Caused by Wuchereria bancrofti and W. malayi, transmitted by the bite of female mosquitoes
- The worms live in lymphatic vessels, usually of the legs, causing chronic inflammation
- Severe swelling of the limbs and sometimes the genital organs
An everyday example. Mass drug administration drives against filariasis in several Indian states give tablets to entire communities to break the cycle of infection.
The substance. The mosquito, not the human, is where Plasmodium reproduces sexually — humans host only the asexual stages.
Life cycle of Plasmodium:
- An infected female Anopheles mosquito injects sporozoites while biting
- The sporozoites multiply asexually in liver cells, which burst and release parasites into the blood
- The parasites multiply inside red blood cells and burst them, releasing a toxin called haemozoin, which causes recurring chills and high fever
- Some parasites develop into sexual stages, gametocytes, in the blood
- A mosquito biting the person takes up gametocytes, and fertilisation and development follow in the mosquito's stomach wall
- New sporozoites move to the mosquito's salivary glands, ready to infect another person
Ascariasis:
- Caused by the roundworm Ascaris, which lives in the intestine
- Eggs passed in faeces contaminate soil, water and vegetables
- Internal bleeding, muscular pain, fever, anaemia and blockage of the intestine
Filariasis (elephantiasis):
- Caused by Wuchereria bancrofti and W. malayi, transmitted by the bite of female mosquitoes
- The worms live in lymphatic vessels, usually of the legs, causing chronic inflammation
- Severe swelling of the limbs and sometimes the genital organs
An everyday example. Mass drug administration drives against filariasis in several Indian states give tablets to entire communities to break the cycle of infection.
The substance. The mosquito, not the human, is where Plasmodium reproduces sexually — humans host only the asexual stages.
What are the types and features of cancer, and how is cancer diagnosed and treated?
Cancer is uncontrolled cell division in which cells lose contact inhibition and form tumours; benign tumours stay in place, while malignant tumours invade tissues and spread by metastasis, and cancer is diagnosed by biopsy, blood tests and imaging and treated by surgery, radiation, chemotherapy and immunotherapy.
Characteristics of cancer cells:
- Divide without control and lose contact inhibition, the signal from neighbouring cells that normally stops division
- Form masses of cells called tumours
- Can invade nearby tissue and travel through the blood to form new tumours elsewhere — metastasis
Types of tumours:
- Benign — stay at their original site and do not spread
- Malignant — grow rapidly, invade surrounding tissue, starve normal cells of nutrients and spread by metastasis
Causes. Carcinogens such as tobacco smoke, X-rays, gamma rays, UV rays and certain chemicals, and oncogenic viruses; normal genes called proto-oncogenes can turn into cancer-causing oncogenes.
Diagnosis:
- Biopsy and histopathology — a thin piece of suspected tissue is stained and examined
- Blood and bone marrow tests — detect leukaemias
- Imaging — radiography, CT and MRI detect internal cancers
- Molecular methods — antibodies against cancer-specific antigens and genetic tests for inherited risk
Treatment:
- Surgery to remove the tumour
- Radiation therapy to kill tumour cells
- Chemotherapy with drugs that kill cancer cells, often with side effects such as hair loss and anaemia
- Immunotherapy with biological response modifiers such as alpha-interferon, which activate the immune system
An everyday example. Health warnings printed on cigarette and bidi packets in India alert people that tobacco smoke is a major carcinogen.
The substance. Not every tumour is cancer — benign tumours do not spread; only malignant tumours count as cancer.
Characteristics of cancer cells:
- Divide without control and lose contact inhibition, the signal from neighbouring cells that normally stops division
- Form masses of cells called tumours
- Can invade nearby tissue and travel through the blood to form new tumours elsewhere — metastasis
Types of tumours:
- Benign — stay at their original site and do not spread
- Malignant — grow rapidly, invade surrounding tissue, starve normal cells of nutrients and spread by metastasis
Causes. Carcinogens such as tobacco smoke, X-rays, gamma rays, UV rays and certain chemicals, and oncogenic viruses; normal genes called proto-oncogenes can turn into cancer-causing oncogenes.
Diagnosis:
- Biopsy and histopathology — a thin piece of suspected tissue is stained and examined
- Blood and bone marrow tests — detect leukaemias
- Imaging — radiography, CT and MRI detect internal cancers
- Molecular methods — antibodies against cancer-specific antigens and genetic tests for inherited risk
Treatment:
- Surgery to remove the tumour
- Radiation therapy to kill tumour cells
- Chemotherapy with drugs that kill cancer cells, often with side effects such as hair loss and anaemia
- Immunotherapy with biological response modifiers such as alpha-interferon, which activate the immune system
An everyday example. Health warnings printed on cigarette and bidi packets in India alert people that tobacco smoke is a major carcinogen.
The substance. Not every tumour is cancer — benign tumours do not spread; only malignant tumours count as cancer.
Exam tip
What earns full marks on human diseases and cancer?
For each disease, write four things — the pathogen, how it spreads, one key symptom and one prevention — so no part of the answer is missed.
- Typhoid: Salmonella typhi, Widal test; pneumonia: Streptococcus pneumoniae
- Malaria: Plasmodium, female Anopheles, haemozoin causing fever
- Filariasis: Wuchereria, lymphatic vessels, swelling of the limbs
- Cancer: loss of contact inhibition; benign versus malignant
The trap. Writing that the male Anopheles spreads malaria. Only the female Anopheles bites humans and transmits Plasmodium.
- Typhoid: Salmonella typhi, Widal test; pneumonia: Streptococcus pneumoniae
- Malaria: Plasmodium, female Anopheles, haemozoin causing fever
- Filariasis: Wuchereria, lymphatic vessels, swelling of the limbs
- Cancer: loss of contact inhibition; benign versus malignant
The trap. Writing that the male Anopheles spreads malaria. Only the female Anopheles bites humans and transmits Plasmodium.
Did you know
Why does malaria fever come and go in a cycle?
In many malaria patients, fever and chills return every few days in a regular rhythm instead of staying constant.
The rhythm follows the parasite's schedule in the blood. Batches of Plasmodium multiply inside red blood cells at the same pace and burst out together, releasing haemozoin all at once, so each synchronised burst brings a fresh wave of chills and fever.
The length of the cycle differs between Plasmodium species, which is one clue that helps tell the species apart.
The rhythm follows the parasite's schedule in the blood. Batches of Plasmodium multiply inside red blood cells at the same pace and burst out together, releasing haemozoin all at once, so each synchronised burst brings a fresh wave of chills and fever.
The length of the cycle differs between Plasmodium species, which is one clue that helps tell the species apart.
Exam relevance
How does NEET test infectious diseases, the malaria life cycle and cancer?
Human Health and Disease is a recurring NEET chapter, and its questions mostly match diseases with pathogens and features.
What gets asked. Pathogens and tests for typhoid, pneumonia, the common cold and amoebiasis, the stages of Plasmodium in humans and mosquitoes, the cause and site of filariasis, benign versus malignant tumours, and methods of cancer detection and treatment.
Question types. Mostly match-the-column and statement-based questions, with diagram-based questions on the Plasmodium life cycle.
Why it matters later. Immunity, vaccines and AIDS follow in the next part of this chapter, and molecular diagnosis returns in Biotechnology and its Applications.
The trap that costs marks. Placing fertilisation of Plasmodium in human blood — gametocytes form in human blood, but fertilisation happens in the mosquito.
What gets asked. Pathogens and tests for typhoid, pneumonia, the common cold and amoebiasis, the stages of Plasmodium in humans and mosquitoes, the cause and site of filariasis, benign versus malignant tumours, and methods of cancer detection and treatment.
Question types. Mostly match-the-column and statement-based questions, with diagram-based questions on the Plasmodium life cycle.
Why it matters later. Immunity, vaccines and AIDS follow in the next part of this chapter, and molecular diagnosis returns in Biotechnology and its Applications.
The trap that costs marks. Placing fertilisation of Plasmodium in human blood — gametocytes form in human blood, but fertilisation happens in the mosquito.
Key takeaways
What must you be able to do from this lesson?
- Infectious diseases: typhoid, pneumonia, the common cold and amoebiasis, with their pathogens, spread and prevention
- Malaria and worms: the Plasmodium life cycle in humans and Anopheles, and ascariasis and filariasis
- Cancer: loss of contact inhibition, benign and malignant tumours, diagnosis by biopsy and imaging, and treatment
Which stage of Plasmodium enters the body in a mosquito bite, and where does it multiply first?
- Malaria and worms: the Plasmodium life cycle in humans and Anopheles, and ascariasis and filariasis
- Cancer: loss of contact inhibition, benign and malignant tumours, diagnosis by biopsy and imaging, and treatment
Which stage of Plasmodium enters the body in a mosquito bite, and where does it multiply first?