BiologyCBSE Class 12 · Human Health and Disease

Common Diseases in Humans and the Life Cycle of Plasmodium

Updated 15 October 2026 · Fact-checked

Pathogens cause infectious diseases: Salmonella typhi causes typhoid, Streptococcus pneumoniae and Haemophilus influenzae pneumonia, rhinoviruses the common cold, Plasmodium malaria, Entamoeba amoebiasis, Ascaris ascariasis, Wuchereria filariasis and fungi ringworm. Plasmodium needs two hosts: it multiplies in human liver cells and RBCs and completes its sexual stages in the female Anopheles mosquito.

Plasmodium needs two hosts: it multiplies in human liver and red blood cells, and bursting red cells cause fever every 3-4 days.

Illustration drawn for QuestionBuddy.

Understand Common Diseases in Humans and Malaria

Health and pathogens. Health is a state of complete physical, mental and social well-being. Disease-causing organisms (pathogens) include bacteria, viruses, fungi, protozoans and helminths (worms).

Disease table (learn row by row):

Disease	Pathogen	Type	Spread	Key symptoms
Typhoid	Salmonella typhi	bacterium	contaminated food and water	sustained high fever (39-40 °C), weakness, stomach pain, constipation, headache, loss of appetite; intestinal perforation in severe cases; Widal test confirms
Pneumonia	Streptococcus pneumoniae, Haemophilus influenzae	bacteria	droplets/aerosols; sharing glasses, utensils	infects the alveoli, which fill with fluid; fever, chills, cough, headache; lips and nails grey to bluish in severe cases
Common cold	rhinoviruses	virus	droplets; contaminated objects (pens, cups, doorknobs, keyboards)	nose and respiratory passage (not lungs): nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness; 3-7 days
Malaria	Plasmodium vivax, P. malariae, P. falciparum	protozoan	bite of infected female Anopheles	chills and high fever recurring every 3-4 days; P. falciparum causes malignant malaria, which can be fatal
Amoebiasis (amoebic dysentery)	Entamoeba histolytica	protozoan	food and water contaminated with faeces; houseflies as mechanical carriers	lives in the large intestine; constipation, abdominal pain and cramps, stools with excess mucus and blood clots
Ascariasis	Ascaris (roundworm)	helminth	eggs in faeces contaminate soil, water, vegetables	internal bleeding, muscular pain, fever, anaemia, blockage of the intestinal passage
Filariasis (elephantiasis)	Wuchereria bancrofti, W. malayi	helminth	bite of female mosquito vectors (NCERT names no genus; mainly Culex in India)	chronic inflammation of organs, usually lymphatic vessels of the lower limbs; gross deformities, including of genital organs
Ringworm	Microsporum, Trichophyton, Epidermophyton	fungi	soil; towels, clothes, combs of infected people	dry, scaly lesions on skin, nails and scalp with intense itching; thrives in warm, moist skin folds (groin, between toes)

Dengue and chikungunya are spread by *Aedes* mosquitoes. The cook Mary Mallon ('Typhoid Mary') was a carrier who spread typhoid for years through the food she prepared.

Life cycle of Plasmodium (two hosts).
1. An infected female Anopheles bites a person and injects sporozoites (the infectious form) with its saliva.
2. Sporozoites reach the liver through the blood and multiply asexually in liver cells, which burst and release parasites into the blood.
3. Parasites invade RBCs, multiply asexually, and burst them. The rupture releases a toxic substance, haemozoin, which causes the chill and high fever recurring every 3-4 days. Released parasites infect new RBCs.
4. Some parasites develop into gametocytes (sexual stages) in RBCs.
5. A female Anopheles biting this person takes up gametocytes with the blood meal.
6. Fertilisation and development take place in the mosquito's intestine (gut); the products multiply to form sporozoites, which migrate to the mosquito's salivary glands.
7. When the mosquito bites another person, sporozoites are injected and the cycle repeats.
So the human is where asexual multiplication happens, and the mosquito is where sexual reproduction (fertilisation) happens; the parasite needs both.

Flow chart: mosquito bite (sporozoites) → liver cells (asexual) → RBCs (asexual, rupture, haemozoin, fever) → gametocytes in RBCs → taken up by female Anopheles → fertilisation in gut → sporozoites → salivary glands → next human.

Prevention and control.

Personal hygiene: keep the body clean; clean drinking water, food, vegetables and fruits.
Public hygiene: safe disposal of waste and excreta; regular cleaning and disinfecting of water tanks, reservoirs, pools and cesspools; hygienic public catering.
Airborne diseases (pneumonia, common cold): avoid close contact with infected persons and their belongings.
Vector-borne diseases (malaria, filariasis, dengue, chikungunya): control or eliminate vectors and breeding places: avoid stagnant water near homes, clean household coolers regularly, use mosquito nets, put fish such as Gambusia (which eat mosquito larvae) in ponds, spray insecticides in ditches, drains and swamps, and fit wire mesh on doors and windows.

Draw and label: life cycle of Plasmodium. Checklist: a circle diagram split into two halves, human (left) and mosquito (right). Human side: sporozoites entering blood → liver cell with multiplying parasites → RBCs bursting → gametocytes. Mosquito side: gametocytes taken up → fertilisation in gut → sporozoites → salivary glands → bite. Labels: sporozoites, liver cells, RBCs, haemozoin release (fever), gametocytes, female Anopheles, fertilisation in gut, salivary glands.

Common Diseases in Humans and Malaria: key facts

Two hosts of Plasmodium: $human: asexual (liver, RBC); female Anopheles: sexual (gut)$; Infectious form = sporozoite; fever toxin = haemozoin.
Vectors: $Anopheles: malaria; female mosquitoes (mainly Culex in India): filariasis; Aedes: dengue, chikungunya$; Houseflies carry Entamoeba mechanically.
Bacterial diseases: $typhoid: S a l m o n e l l a t y p hi; pneumonia: S t r e pt ococc u s p n e u m o nia e, H a e m o p hi l u s in f l u e n z a e$; Widal test confirms typhoid.
Helminth and fungal diseases: $ascariasis: A sc a r i s; filariasis: W u c h er er ia; ringworm: M i cr os p or u m, T r i c h o p h y t o n, E p i d er m o p h y t o n$; Filarial worms: W. bancrofti, W. malayi.
Organ affected: $pneumonia: alveoli; common cold: nose and air passage; amoebiasis: large intestine$; Typhoid bacteria enter the small intestine and spread through blood.

How to answer Common Diseases in Humans and Malaria questions

Disease questions are usually table-completion, symptom-matching, life-cycle or prevention questions.

1Classify the pathogen first (bacterium, virus, protozoan, helminth, fungus); it narrows the disease.
2Match the organ and mode of spread: alveoli and droplets (pneumonia); intestine and contaminated food/water (typhoid, amoebiasis, ascariasis); mosquito bite (malaria, filariasis).
3For 'which disease' cases, use the distinguishing symptom: sustained fever + Widal (typhoid); bluish lips and nails (pneumonia); fever recurring every 3-4 days with chills (malaria).
4For the Plasmodium life cycle, state the host and process at each stage, and name sporozoites, haemozoin and gametocytes.
5For prevention, separate personal hygiene, public hygiene and vector control, and give specific measures (Gambusia, mosquito nets, no stagnant water).
6If asked why a parasite needs two hosts, say: asexual multiplication in humans, sexual reproduction (fertilisation) in the mosquito.

Common Diseases in Humans and Malaria in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Vector control and the Plasmodium life cycle

Suppose a housing society reports several malaria cases during the monsoon. The resident welfare association finds water collecting in desert coolers, an ornamental pond and blocked roadside drains.

Explain why these sites matter, and suggest a plan of four measures that would break the malaria cycle, saying which stage each measure targets.

Try it first, then show the solutionSolution

Female Anopheles mosquitoes breed in stagnant water; more mosquitoes mean more bites carrying sporozoites to people and gametocytes from patients.
Empty and clean coolers regularly and clear blocked drains: removes breeding places (larvae).
Introduce Gambusia fish into the pond: they eat mosquito larvae.
Use mosquito nets and wire mesh on doors and windows: prevents bites, so sporozoites are not injected and gametocytes are not taken up.
Spray insecticides in drains and swamps: kills adult mosquitoes; treat patients so they stop being a source of gametocytes.

Answer: Stagnant water breeds Anopheles; remove breeding sites, add Gambusia, use nets and mesh, spray insecticides, and treat patients to break the human-mosquito cycle.

Memory hook: No stagnant water, no mosquito; no mosquito, no malaria.

Tricks to remember Common Diseases in Humans and Malaria

Short memory aids. Say each one aloud once, then use it on a question the same day.

Bacteria in T and P
Typhoid and Pneumonia are bacterial; the Widal test is for typhoid.
Use it when: Classifying diseases.
Know the vector
Anopheles carries malaria; Aedes carries dengue and chikungunya; filarial worms ride in other female mosquitoes (mainly Culex in India).
Use it when: Matching vectors.
S-L-R-G-S
Sporozoite → Liver → RBC → Gametocyte → Salivary gland.
Use it when: Order of the Plasmodium life cycle.

Common Diseases in Humans and Malaria: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Complete the table · 2 marks · Very short answerBoard level

Complete: Pneumonia - Streptococcus - (A); Typhoid - (B) - sustained high fever; (C) - rhinovirus - nasal congestion; Ascariasis - (D) - blockage of intestinal passage.

Try it first, then show the solutionSolution

(A) fever, chills, cough, headache; alveoli fill with fluid. (B) Salmonella typhi (1 mark).
(C) common cold. (D) Ascaris (1 mark).

Answer: (A) fever, chills, cough (fluid in alveoli) (B) Salmonella typhi (C) common cold (D) Ascaris.

Example 2: Two hosts · 3 marks · Short answerBoard level

Plasmodium needs both a mosquito and a human host to complete its life cycle. Explain.

Try it first, then show the solutionSolution

Sporozoites injected by the female Anopheles multiply asexually in human liver cells and then in RBCs; RBC rupture releases haemozoin, causing fever (1 mark).
Gametocytes formed in human RBCs cannot fertilise in humans; they are taken up by a female Anopheles during a blood meal (1 mark).
Fertilisation and development occur in the mosquito's gut, forming sporozoites that move to the salivary glands and are injected into a new human; so the cycle needs both hosts (1 mark).

Answer: Asexual stages in humans, sexual stages in the mosquito; each host completes a part the other cannot.

Example 3: Fever with a pattern · 3 marks · Short answerHarder board level

Suppose a patient's temperature chart shows bouts of chills and fever that come back every three days, separated by near-normal temperature. (a) Name the likely disease and pathogen. (b) What causes the fever bouts? (c) Why are they periodic?

Try it first, then show the solutionSolution

(a) Malaria, caused by Plasmodium (1 mark).
(b) When infected RBCs rupture they release the toxin haemozoin, which causes chills and high fever (1 mark).
(c) The parasites multiply inside RBCs in a cycle and burst them together at regular intervals, releasing new parasites that infect fresh RBCs; so fever recurs every 3-4 days (1 mark).

Answer: Malaria (Plasmodium); haemozoin from bursting RBCs; synchronised RBC cycles make fever periodic.

Example 4: Typhoid, pneumonia or malaria? · 3 marks · Short answerHarder board level

A person has high fever. Which symptoms or tests would help you tell whether it is typhoid, pneumonia or malaria?

Try it first, then show the solutionSolution

Typhoid: sustained high fever (39-40 °C) with weakness, stomach pain, constipation, headache and loss of appetite; confirmed by the Widal test (1 mark).
Pneumonia: fever with chills, cough and headache, breathing difficulty because alveoli fill with fluid; in severe cases lips and nails turn grey to bluish (1 mark).
Malaria: chills and high fever recurring every 3-4 days, after a mosquito bite; caused by Plasmodium (1 mark).

Answer: Sustained fever + Widal (typhoid); cough, breathing trouble, bluish lips (pneumonia); periodic fever with chills (malaria).

Example 5: Filariasis · 3 marks · Short answerHarder board level

Name the two species of filarial worms. How do they affect the body, and how are they transmitted? Suggest two control measures.

Try it first, then show the solutionSolution

Wuchereria bancrofti and W. malayi (1 mark).
They live for many years in lymphatic vessels (usually of the lower limbs), causing slowly developing chronic inflammation and gross deformities (elephantiasis); genital organs may also be affected. They are transmitted by the bite of female mosquito vectors (mainly Culex in India) (1 mark).
Control: avoid stagnant water and clean drains; use mosquito nets and wire mesh; Gambusia in ponds; insecticide spraying (any two) (1 mark).

Answer: W. bancrofti, W. malayi; lymphatic inflammation and elephantiasis; female mosquitoes (mainly Culex); remove breeding sites, nets.

Example 6: Why amoebiasis spreads in summer camps · 3 marks · Short answerHOTS / competency

Suppose many children at a summer camp develop abdominal cramps and stools with mucus and blood. The camp kitchen is open and flies are common. (a) Name the likely disease and pathogen. (b) Explain the role of houseflies. (c) Suggest three hygiene measures.

Try it first, then show the solutionSolution

(a) Amoebiasis (amoebic dysentery), caused by Entamoeba histolytica, a protozoan of the large intestine (1 mark).
(b) Houseflies are mechanical carriers: they carry the parasite from the faeces of infected persons to food, contaminating it (1 mark).
(c) Cover food and keep flies out; safe disposal of excreta away from the kitchen; boiled or clean drinking water and washing of hands, vegetables and fruits (1 mark).

Answer: Amoebiasis (Entamoeba); flies carry it from faeces to food; cover food, dispose of excreta safely, clean water and hand-washing.

Where students lose marks in Common Diseases in Humans and Malaria

The slips examiners see most often, and what to write instead.

Mistake: Saying malaria is caused by mosquitoes
Vector and pathogen are confused.
Wrong working
Malaria is caused by the Anopheles mosquito.
Right working
Malaria is caused by Plasmodium; the female Anopheles is the vector.
Do this: Always name the pathogen and the vector separately.
Usually costs: 1 mark.
Mistake: Fertilisation of Plasmodium in humans
Students think gametocytes fuse in human blood.
Wrong working
Gametocytes fertilise in human RBCs.
Right working
Gametocytes form in human RBCs; fertilisation occurs in the mosquito's gut.
Do this: Human = asexual; mosquito = sexual.
Usually costs: 1 mark.
Mistake: Pneumonia in the bronchi
Organ not learnt precisely.
Wrong working
Pneumonia infects the nasal passage.
Right working
Pneumonia infects the alveoli, which fill with fluid; the common cold affects the nose and respiratory passage, not lungs.
Do this: Learn the organ column of the table.
Usually costs: 1 mark.
Mistake: Mixing up the two helminths
Both Ascaris and Wuchereria are worms.
Wrong working
Filariasis is caused by Ascaris.
Right working
Filariasis (elephantiasis) is caused by Wuchereria (W. bancrofti, W. malayi), spread by female mosquitoes; Ascaris causes ascariasis.
Do this: Wuchereria swells the limbs; Ascaris blocks the intestine.
Usually costs: 1 mark.

Exam tips

Write scientific names in italics style with genus capitalised (Salmonella typhi, Plasmodium vivax).
In life-cycle answers, mention both hosts, sporozoites, liver, RBCs, haemozoin, gametocytes and salivary glands.
Give the Widal test whenever typhoid diagnosis is asked.
For prevention, give specific measures (Gambusia, coolers, nets, wire mesh), not just 'keep surroundings clean'.
Only the diseases named in the syllabus are tested: malaria, dengue, chikungunya, filariasis, ascariasis, typhoid, pneumonia, common cold, amoebiasis, ringworm.

Practice questions on Common Diseases in Humans and Malaria

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The infectious stage of Plasmodium that enters the human body is the:
(a) gametocyte (b) sporozoite (c) merozoite in RBC (d) trophozoite
Hint
Injected with mosquito saliva.
Answer
(b) sporozoite.
Q2 · 1 mark · MCQBoard level
Which pairing is mismatched?
(a) Common cold - rhinovirus (b) Ringworm - Trichophyton (c) Filariasis - Ascaris (d) Typhoid - Salmonella typhi
Hint
Filarial worms.
Answer
(c). Filariasis is caused by Wuchereria; Ascaris causes ascariasis.
Q3 · 2 marks · Very short answerHarder board level
Why do ringworm infections commonly appear in the groin and between the toes? How is ringworm acquired?
Hint
Conditions fungi like.
Answer
The fungi (Microsporum, Trichophyton, Epidermophyton) thrive in heat and moisture, which skin folds such as the groin and between toes provide. Ringworm is acquired from soil or by using towels, clothes or combs of infected people.
Q4 · 3 marks · Short answerHOTS / competency
Suppose a town keeps reporting dengue cases although most families sleep under mosquito nets. Name the vector of dengue. Explain why eliminating breeding places works better than relying on nets alone, and give three such measures.
Hint
Nets protect people; breeding control removes mosquitoes.
Answer
Dengue is spread by Aedes mosquitoes. Nets protect only the people under them while they sleep, but the mosquito population stays the same and people are bitten at other times and places; removing breeding places cuts the number of mosquitoes itself, protecting everyone. Measures: avoid stagnant water around homes and clean household coolers regularly; introduce Gambusia fish (which eat larvae) in ponds; spray insecticides in ditches, drains and swamps (also wire mesh on doors and windows).
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a passage describes how mosquitoes affect health: one genus transmits a protozoan that destroys RBCs; another transmits a worm that blocks lymphatic vessels; a third transmits viral fevers.
(a) Name the protozoan disease and its vector. (1)
(b) Name the worm and the disease it causes. (1)
(c) Give two methods of controlling mosquito larvae and explain one of them. (2)
Hint
Anopheles, Wuchereria, Gambusia.
Answer
(a) Malaria; female Anopheles. (b) Wuchereria (W. bancrofti, W. malayi); filariasis (elephantiasis). (c) Introducing Gambusia fish in ponds and removing stagnant water (or spraying insecticides in ditches and swamps). Gambusia feeds on mosquito larvae, so fewer adults emerge.
Q6 · 3 marks · Short answerHarder board level
Describe the life cycle of Plasmodium in the human body, naming the cells it infects and the substance that causes fever.
Hint
Liver first, then RBCs.
Answer
Sporozoites injected by an infected female Anopheles reach the liver and multiply asexually in liver cells, which burst and release parasites into the blood. These invade RBCs, multiply asexually and rupture them, releasing haemozoin, which causes chills and high fever every 3-4 days; released parasites infect new RBCs. Some develop into gametocytes, which are taken up by a mosquito.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which test confirms typhoid?Show answer
The Widal test.
2.Which species causes malignant malaria?Show answer
Plasmodium falciparum.
3.What toxic substance causes malarial fever?Show answer
Haemozoin.
4.Where do Plasmodium sporozoites collect in the mosquito?Show answer
In the salivary glands.
5.Which fish is used to control mosquito larvae?Show answer
Gambusia.
6.Name the vector of dengue and chikungunya.Show answer
Aedes mosquitoes.

Common Diseases in Humans and Malaria: frequently asked questions

What is the life cycle of Plasmodium?

Sporozoites from a female Anopheles bite multiply in human liver cells, then in RBCs (bursting them and causing fever); some form gametocytes, which a mosquito takes up; fertilisation in the mosquito's gut produces sporozoites that move to its salivary glands.

Why does malaria cause recurring fever?

Infected RBCs burst at regular intervals and release haemozoin, which causes chills and high fever every 3-4 days.

What causes typhoid and how is it diagnosed?

Salmonella typhi, taken in with contaminated food or water; it is confirmed by the Widal test.

How can mosquito-borne diseases be controlled?

By removing stagnant water, cleaning coolers, using mosquito nets and wire mesh, putting Gambusia fish in ponds and spraying insecticides in ditches and swamps.

What is the difference between pneumonia and the common cold?

Pneumonia is bacterial and infects the alveoli of the lungs; the common cold is caused by rhinoviruses and affects the nose and respiratory passage, not the lungs.