BiologyISC Class 12 · Human Health and Diseases

Common Diseases in Humans and Malaria: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Pathogens cause infectious diseases: viruses (common cold, dengue, chikungunya), bacteria (typhoid, pneumonia, diphtheria, plague), protozoans (amoebiasis, malaria), helminths (ascariasis, filariasis) and fungi (ringworm). Malaria is caused by Plasmodium and spread by the female Anopheles mosquito; rupture of RBCs releases haemozoin, causing chill and high fever every three to four days.

Understand Common Diseases in Humans and Malaria

Pathogens and parasites. Disease-causing organisms (bacteria, viruses, fungi, protozoans, helminths) are pathogens. Most parasites are pathogens because they harm the host by living in or on it. A pathogen must adapt to the host: gut pathogens survive stomach acid and digestive enzymes.

Master table (learn with binomial names, italicised or underlined in handwriting).

Disease	Group	Causative agent	Transmission	Key symptoms
Common cold	virus	Rhinoviruses	droplets from coughs and sneezes; contaminated objects (pens, cups, doorknobs, keyboards)	nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness for 3-7 days; infects nose and respiratory passage, not lungs
Dengue	virus	dengue virus	bite of female Aedes mosquito	high fever, severe headache, pain behind the eyes, joint and muscle pain, rash, low platelets
Chikungunya	virus	chikungunya virus	Aedes mosquito	fever, severe and often prolonged joint pain
Typhoid	bacterium	Salmonella typhi	contaminated food and water; enters the small intestine, spreads by blood	sustained high fever (39-40 °C), weakness, stomach pain, constipation, headache, loss of appetite; intestinal perforation in severe cases; confirmed by Widal test
Pneumonia	bacteria	Streptococcus pneumoniae, Haemophilus influenzae	inhaling droplets or aerosols; sharing glasses and utensils	alveoli fill with fluid; fever, chills, cough, headache; lips and nails grey to bluish in severe cases
Diphtheria	bacterium	Corynebacterium diphtheriae	droplets	membrane in the throat, fever, difficulty in breathing; DPT vaccine
Plague	bacterium	Yersinia pestis	bite of rat flea	high fever, swollen lymph nodes (buboes)
Amoebiasis (amoebic dysentery)	protozoan	Entamoeba histolytica (large intestine)	food and water contaminated with faeces; houseflies are mechanical carriers	constipation, abdominal pain and cramps, stools with excess mucus and blood clots
Malaria	protozoan	Plasmodium vivax, P. malariae, P. falciparum	bite of infected female Anopheles	chill and high fever recurring every 3-4 days
Ascariasis	helminth (roundworm)	Ascaris lumbricoides	eggs in faeces contaminate soil, water, vegetables and fruits	internal bleeding, muscular pain, fever, anaemia, blockage of the intestine
Filariasis (elephantiasis)	helminth (filarial worm)	Wuchereria bancrofti, W. malayi	bite of female Culex mosquito	chronic inflammation of lymphatic vessels of lower limbs; swelling and deformity of limbs and genitals
Ringworm	fungi	Microsporum, Trichophyton, Epidermophyton	soil; towels, clothes or combs of infected people	dry, scaly lesions on skin, nails and scalp with intense itching; thrive in warm, moist skin folds (groin, between toes)

Malaria and the life cycle of Plasmodium (graphic outline). Malignant malaria by P. falciparum is the most serious and can be fatal. The parasite needs two hosts: human and female Anopheles mosquito, which is also the vector.
In the human:
1. Infected female Anopheles bites and injects sporozoites (infectious form).
2. Sporozoites reach the liver and multiply in liver cells (asexual).
3. Released parasites attack RBCs, multiply asexually, and burst the RBCs. Rupture releases the toxin haemozoin → chill and high fever recurring every three to four days.
4. Some parasites in RBCs develop into gametocytes (male and female).
In the mosquito:
5. A female Anopheles biting the patient takes up gametocytes with blood.
6. Fertilisation and development take place in the mosquito's stomach (gut) wall (sexual stage).
7. Many sporozoites form and move to the salivary glands, ready to be injected at the next bite.
The sexual phase occurs in the mosquito, so it is the definitive (primary) host; the human, with only asexual multiplication, is the intermediate (secondary) host.

Diagram checklist (Plasmodium flow chart). Two loops joined at the bite: human loop (sporozoites → liver cells → RBCs → rupture, haemozoin → gametocytes) and mosquito loop (gametocytes → fertilisation in gut → sporozoites → salivary glands). Label 'asexual in human', 'sexual in mosquito', 'female Anopheles', 'haemozoin'.

Prevention and control.

Personal hygiene: clean body, clean drinking water, food, vegetables and fruits.
Public hygiene: safe disposal of garbage and excreta; regular cleaning and disinfecting of tanks, pools, cesspools and other water reservoirs; hygienic public catering. Essential for food- and water-borne diseases (typhoid, amoebiasis, ascariasis).
Air-borne diseases (pneumonia, common cold): avoid close contact with infected persons and their belongings.
Vector-borne diseases (malaria, filariasis, dengue, chikungunya): avoid stagnant water near homes, clean coolers regularly, use mosquito nets and screens, spray insecticides, introduce fish such as Gambusia that eat mosquito larvae.
Vaccination (for example DPT for diphtheria) and antibiotics for bacterial infections.

Common Diseases in Humans and Malaria: key facts

Mosquito and disease: Female Anopheles malaria (Plasmodium)
Female Culex filariasis (Wuchereria)
Aedes dengue and chikungunya (viruses); Always the female mosquito.
Plasmodium in humans: 1.Bite injects sporozoites
2.multiply in liver cells
3.attack and multiply in RBCs
4.RBCs burst, haemozoin causes chill and fever
5.gametocytes form in RBCs; Asexual phase.
Plasmodium in mosquito: 1.Mosquito takes up gametocytes
2.fertilisation and development in the gut
3.sporozoites formed
4.stored in salivary glands
5.injected at the next bite; Sexual phase.
Clue to disease: Chill and fever every 3-4 days malaria
Sustained 39-40 °C fever typhoid, confirmed by Widal test
Swollen lower limbs filariasis
Itchy, dry, scaly patches ringworm
Mucus and blood clots in stools amoebiasis; Widal is for typhoid only; AIDS is diagnosed by ELISA.

Female Anopheles	malaria (Plasmodium)
Female Culex	filariasis (Wuchereria)
Aedes	dengue and chikungunya (viruses)

Chill and fever every 3-4 days	malaria
Sustained 39-40 °C fever	typhoid, confirmed by Widal test
Swollen lower limbs	filariasis
Itchy, dry, scaly patches	ringworm
Mucus and blood clots in stools	amoebiasis

How to answer Common Diseases in Humans and Malaria questions

ISC asks for causative agents with scientific names, mode of transmission, symptoms and prevention (often in a table), the Plasmodium life cycle as a graphic outline, and clue-based identification.

1Give the full binomial name with a capital genus and small species, underlined in handwriting: Wuchereria bancrofti, Salmonella typhi.
2Name the vector exactly: female Culex for filariasis, female Anopheles for malaria, Aedes for dengue and chikungunya.
3For transmission, give the route (food and water, droplets, vector bite, contact) rather than 'germs'.
4For the Plasmodium cycle, draw two loops (human, mosquito) and name sporozoites, liver, RBCs, haemozoin, gametocytes, gut, salivary glands.
5For prevention, match the measure to the route: hygiene for faecal-oral, vector control for mosquito-borne, avoiding contact for air-borne.
6In clue questions, pick the defining symptom: lesions with itching (ringworm), swollen legs (filariasis), periodic fever (malaria).

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 for malaria

Suppose a village near paddy fields reports many cases of fever with chills that return every few days, mainly after the rains, and blood smears show a protozoan inside red blood cells.

Identify the disease and suggest a plan to cut transmission.

Try it first, then show the solutionSolution

Periodic chill and fever with parasites in RBCs indicates malaria (Plasmodium), spread by female Anopheles.
Rains create stagnant water where Anopheles breeds, so cases rise.
Drain or cover stagnant water, clean coolers and containers weekly, introduce Gambusia fish into ponds and paddy channels.
Use bed nets and screens, spray insecticides where needed, and treat patients early so mosquitoes cannot pick up gametocytes.

Answer: Malaria; control the female Anopheles (breeding sites, Gambusia, nets, spraying) and treat patients early to break the human-mosquito cycle.

Memory hook: No standing water, no Anopheles, 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.

A-C-A mosquitoes
Anopheles for Ague (malaria fever), Culex for Chronic swelling (filariasis), Aedes for Aches (dengue, chikungunya).
Use it when: Matching vector to disease.
Sporozoites in, gametocytes out
The mosquito gives sporozoites to humans and takes gametocytes from humans.
Use it when: Plasmodium life cycle.
Ring of three fungi
My Tiny Enemies: Microsporum, Trichophyton, Epidermophyton.
Use it when: Ringworm agents.

Common Diseases in Humans and Malaria: solved examples

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

Example 1: Agent and transmission · 2 marks · Short answerBoard level

For (i) ascariasis and (ii) pneumonia, name the pathogen by its binomial name and state how it reaches a new host.

Try it first, then show the solutionSolution

(i) Ascaris lumbricoides; eggs passed in faeces contaminate soil, water and vegetables, which are taken in with food or water (1 mark).
(ii) Streptococcus pneumoniae (or Haemophilus influenzae); inhaling droplets or aerosols from an infected person or sharing glasses and utensils (1 mark).

Answer: Ascaris lumbricoides via contaminated food and water; Streptococcus pneumoniae via droplets.

Example 2: Plasmodium outline · 3 marks · Short answerBoard level

Draw a flow chart of the life cycle of Plasmodium, showing the stages in the human and in the mosquito.

Try it first, then show the solutionSolution

Human: bite of infected female Anopheles injects sporozoites → multiply in liver cells → attack RBCs, multiply, RBCs rupture releasing haemozoin (fever) (1 mark).
→ some form gametocytes in RBCs (½ mark).
Mosquito: female Anopheles takes up gametocytes → fertilisation and development in the gut → sporozoites → salivary glands → injected at next bite (1½ marks).

Answer: Two-loop flow chart: asexual in human, sexual in mosquito.

Example 3: Why the fever is periodic · 2 marks · Short answerHarder board level

Explain why a malaria patient suffers chill and high fever at intervals of three to four days, and why P. falciparum malaria is called malignant.

Try it first, then show the solutionSolution

The parasites multiply inside RBCs in step; at the end of each round the RBCs burst together and release haemozoin, causing chill and high fever; the cycle repeats every three to four days (1 mark).
P. falciparum causes the most serious form, which can be fatal, so it is called malignant malaria (1 mark).

Answer: Synchronised RBC rupture releases haemozoin; falciparum malaria can kill.

Example 4: Hosts of Plasmodium · 3 marks · Short answerHarder board level

Which is the definitive host and which the intermediate host of Plasmodium? Give reasons. Why is the female Anopheles also called a vector?

Try it first, then show the solutionSolution

Definitive (primary) host: female Anopheles, because fertilisation (the sexual phase) happens in it (1 mark).
Intermediate (secondary) host: human, where only asexual multiplication occurs in liver cells and RBCs (1 mark).
The mosquito carries sporozoites in its salivary glands and transmits them to humans at each bite, so it is also the vector (1 mark).

Answer: Mosquito definitive (sexual phase); human intermediate (asexual); mosquito transmits, so it is the vector.

Example 5: Faecal-oral diseases · 3 marks · Short answerHarder board level

Typhoid, amoebiasis and ascariasis are common where sanitation is poor. Give the causative agent of each, explain the common route of spread and state three public hygiene measures that control all three.

Try it first, then show the solutionSolution

Agents: Salmonella typhi; Entamoeba histolytica; Ascaris lumbricoides (1 mark).
All spread through food and water contaminated with faeces of infected people; houseflies carry amoebic cysts to food (1 mark).
Measures: proper disposal of waste and excreta; periodic cleaning and disinfection of water reservoirs and tanks; standard hygiene in public catering (1 mark).

Answer: Faecal-oral route; control by sanitation, safe water and catering hygiene.

Example 6: A hostel outbreak · 4 marks · Short answerHOTS / competency

Suppose in a hostel, student P has itchy, circular, scaly patches between the toes; student Q has sustained fever of 39-40 °C with constipation and stomach pain; student R has fever and severe joint pain after many mosquito bites in daytime. (a) Suggest the likely disease and causative group for each. (b) Name the confirmatory test for Q. (c) Give one specific preventive step for each.

Try it first, then show the solutionSolution

(a) P: ringworm (fungi, e.g. Trichophyton); Q: typhoid (bacterium Salmonella typhi); R: chikungunya or dengue (virus, Aedes-borne) (2 marks).
(b) Widal test (1 mark).
(c) P: keep skin folds dry, do not share towels or footwear; Q: safe drinking water and hygienic food; R: remove stagnant water in coolers and containers, use repellents and nets (1 mark).

Answer: Ringworm, typhoid, chikungunya/dengue; Widal; dry skin, safe food and water, vector control.

Where students lose marks in Common Diseases in Humans and Malaria

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

Mistake: Wrong vector for filariasis
In 2024 examiners reported Anopheles, Aedes or just 'mosquito' instead of the female Culex mosquito.
Do this: Learn the three mosquitoes with their diseases.
Usually costs: 1 mark.
Mistake: Misspelt or general names for agents
Examiners (2024) noted misspelt organism names, binomial rules ignored, and general terms like 'bacteria' or 'virus' given instead of the scientific name.
Do this: Write the genus with a capital and species in small letters, underlined: Salmonella typhi.
Usually costs: 1 mark per disease.
Mistake: Drawing the Plasmodium cycle for another question
Examiners (2025) found the Plasmodium life cycle drawn when the retrovirus cycle was asked.
Do this: Read the organism in the question; Plasmodium has two hosts, a retrovirus one cell.
Usually costs: Whole answer.
Mistake: Mixing up the hosts
Calling the human the definitive host is a frequent error.
Do this: Sexual phase in mosquito = definitive host.
Usually costs: 1 mark.

Exam tips

Tabulate agent, transmission, symptom and prevention whenever two or more diseases are asked.
Always write 'female' before the mosquito name.
Use 'sporozoites' and 'gametocytes' in the right direction of transfer.
Give Widal only for typhoid.
Match prevention to the route of spread.

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
Filariasis is transmitted by:
(a) female Anopheles (b) female Culex (c) Aedes (d) housefly
Hint
Not malaria's vector.
Answer
(b) female Culex.
Q2 · 2 marks · Short answerBoard level
Name the toxin released in malaria and state its effect. Which stage of Plasmodium enters the human body?
Hint
Rupture of RBCs.
Answer
Haemozoin, released when infected RBCs rupture; it causes chill and high fever recurring every three to four days. Sporozoites enter the human body.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The common cold does not usually cause pneumonia-like fluid in the alveoli.
Reason (R): Rhinoviruses attack the nasal and respiratory passages and do not reach the lungs.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Where do rhinoviruses act?
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
State the causative agents of ringworm and explain why the disease is common in the groin and between the toes.
Hint
Heat and moisture.
Answer
Fungi of the genera Microsporum, Trichophyton and Epidermophyton. Heat and moisture help these fungi grow, and skin folds such as the groin and between the toes stay warm and moist.
Q5 · 2 marks · Short answerHarder board level
Why are houseflies important in the spread of amoebiasis? Give two symptoms of the disease.
Hint
Mechanical carriers.
Answer
Houseflies act as mechanical carriers, transferring the parasite from the faeces of infected people to food. Symptoms: constipation, abdominal pain and cramps, stools with excess mucus and blood clots (any two).
Q6 · 2 marks · Short answerHarder board level
Explain how filarial worms cause elephantiasis and why the disease is called chronic.
Hint
Lymphatic vessels.
Answer
Wuchereria worms live for many years in lymphatic vessels, usually of the lower limbs, causing slowly developing inflammation and blockage, so the limbs (and often genitals) swell and become deformed. It develops slowly and lasts for years, so it is chronic.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a district reduces mosquito breeding sharply but continues to see some malaria cases from travellers returning from other regions. Explain how a single untreated traveller can restart local transmission and what step prevents it.
Hint
Gametocytes in blood.
Answer
The traveller's blood carries gametocytes; a local female Anopheles biting him takes them up; fertilisation and development in the mosquito produce sporozoites in its salivary glands, which it injects into healthy people. Early testing and complete treatment of returning travellers (and continued vector control) prevent this.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a cook who feels perfectly well is found to carry Salmonella typhi, and several customers of her eatery fall ill with sustained high fever and stomach pain.
(a) Name the disease and the test that confirms it. (1)
(b) How did the customers get infected though the cook was not ill? (1)
(c) Give two measures the eatery must follow. (1)
Hint
A carrier, like the case of Mary Mallon.
Answer
(a) Typhoid; Widal test. (b) She is a carrier: she harbours the bacteria without symptoms and contaminated the food she prepared. (c) Screening and treatment of food handlers; hand washing, clean water and hygienic food preparation (standard hygiene in public catering).

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 Plasmodium species causes malignant malaria?Show answer
Plasmodium falciparum.
2.Where does fertilisation of Plasmodium take place?Show answer
In the gut (stomach) of the female Anopheles mosquito.
3.Causative agent of plague?Show answer
Yersinia pestis (spread by the rat flea).
4.Which part of the respiratory system do rhinoviruses infect?Show answer
The nose and respiratory passage, not the lungs.
5.Name a fish used to control mosquito larvae.Show answer
Gambusia.
6.Which test confirms typhoid?Show answer
Widal test.

Common Diseases in Humans and Malaria: frequently asked questions

What causes malaria and how is it spread?

Malaria is caused by species of Plasmodium (P. vivax, P. malariae, P. falciparum) and spread by the bite of an infected female Anopheles mosquito.

Why does malaria fever come back every few days?

Infected RBCs burst together at the end of each round of multiplication and release haemozoin, which causes chill and high fever every three to four days.

What is the difference between Culex and Anopheles diseases?

Female Culex spreads filariasis (Wuchereria); female Anopheles spreads malaria (Plasmodium).

Which diseases are controlled mainly by hygiene?

Food- and water-borne diseases such as typhoid, amoebiasis and ascariasis.

Is ringworm caused by a worm?

No. It is a fungal infection caused by Microsporum, Trichophyton or Epidermophyton.