Section 14 — Solved Examples
This is the dedicated problem set for Chapter 7 (Human Health and Disease). The 30 worked examples below are arranged in three tiers — concept checks (pathogen–disease matching, definitions, vector logic), application & scenarios (case-style situations on malaria, immunity, AIDS and cancer), and analytical & multi-concept (reasoning problems that link immunity, transmission and prevention across the whole chapter).
Unlike a straight glossary, most of these ask you to identify, predict and justify, because that is exactly how a disease question earns marks in a Board or NEET paper. Naming the pathogen is rarely enough; you are usually asked why a symptom appears, how transmission happens, or what the immune system is doing.
How to use this section
- Concept Checks (Q1–Q10): Quick recall of pathogens, symptoms and key terms. If you stumble on more than one or two, revisit Sections 2–5.
- Application & Scenarios (Q11–Q20): The workhorse 2- and 3-mark questions — life cycles, immunity types, HIV mechanism, tumour behaviour.
- Analytical & Multi-Concept (Q21–Q30): Multi-step reasoning that ties several topics together, the kind that separates a good answer from a full-mark answer.
Total target time: around an hour for a complete revision sweep.
Note: This section is for practice and revision only — there is no quiz at the end. Treat each answer as a model of how much detail an examiner expects.
Memory Capsule — Facts Worth Locking In
Before working through the problems, fix these ten high-yield facts firmly in mind:
| # | Fact | Where it is tested |
|---|---|---|
| 1 | Typhoid — Salmonella typhi; confirmed by the Widal test; classic carrier was Typhoid Mary | Pathogen matching |
| 2 | Pneumonia — Streptococcus pneumoniae and Haemophilus influenzae; infects the alveoli | Symptom logic |
| 3 | Malaria — Plasmodium; female Anopheles is the vector; haemozoin causes the recurring chill and fever | Life-cycle questions |
| 4 | Innate immunity = non-specific, present from birth; four barriers (physical, physiological, cellular, cytokine) | Immunity types |
| 5 | B-cells make antibodies (humoral); T-cells run cell-mediated immunity and cause graft rejection | Response types |
| 6 | Antibody = H2L2 — two heavy, two light chains; classes include IgA, IgM, IgE, IgG | Structure questions |
| 7 | Active immunity — body makes its own antibodies (slow); passive — ready-made antibodies given (fast), e.g. colostrum, antivenom | Compare-and-contrast |
| 8 | HIV is a retrovirus; uses reverse transcriptase; destroys helper T-cells; diagnosed by ELISA | AIDS mechanism |
| 9 | Cancer cells lose contact inhibition; malignant tumours show metastasis; agents that cause it are carcinogens | Cancer questions |
| 10 | Abused drugs — opioids (heroin, from Papaver somniferum), cannabinoids (from Cannabis sativa), cocaine (from Erythroxylum coca, blocks dopamine transport) | Drug source and action |
Pro tip: For almost every disease question, three things earn marks — the pathogen, the mode of transmission, and one prevention or control measure. Answer all three even when only asked for one.
Concept Checks (Q1–Q10)
Q1. Name the pathogen that causes typhoid and the test used to confirm the disease.
Answer: Typhoid is caused by the bacterium Salmonella typhi. It is confirmed by the Widal test. The pathogen enters through contaminated food and water, reaches the small intestine, and spreads to other organs through the blood, producing sustained high fever, weakness, stomach pain and loss of appetite.
Q2. Which organ of the body does pneumonia primarily affect, and name the two bacteria responsible.
Answer: Pneumonia affects the alveoli (the air-filled sacs) of the lungs, which fill with fluid and make breathing difficult. The bacteria responsible are Streptococcus pneumoniae and Haemophilus influenzae. In severe cases the lips and finger nails may turn grey to bluish because of poor oxygenation.
Q3. Which group of viruses causes the common cold, and which part of the body do they infect?
Answer: The common cold is caused by rhinoviruses. They infect the nose and the respiratory passage but not the lungs, producing nasal congestion, sore throat, cough and tiredness that usually last three to seven days.
Q4. Name the protozoan that causes amoebiasis, the organ it infects, and its mechanical carrier.
Answer: Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, which lives in the large intestine. Houseflies act as mechanical carriers, transferring the parasite from the faeces of an infected person to food, so contaminated food and water are the main source of infection.
Q5. Match each helminth with the disease it causes: (a) Ascaris (b) Wuchereria.
Answer: (a) Ascaris (the common round worm) causes ascariasis, with symptoms such as internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage. (b) Wuchereria (the filarial worm) causes elephantiasis (filariasis), a chronic inflammation usually of the lymphatic vessels of the lower limbs.
Q6. What are the two broad types of immunity, and which one is present from birth?
Answer: The two types are innate immunity and acquired immunity. Innate immunity is present from birth; it is a non-specific defence that works through four kinds of barriers — physical, physiological, cellular and cytokine. Acquired immunity develops after exposure to a pathogen and is specific.
Q7. State the H2L2 formula of an antibody and identify what the letters mean.
Answer: An antibody molecule is written as H2L2, meaning it has two heavy (H) chains and two light (L) chains. Four classes commonly named are IgA, IgM, IgE and IgG. Because antibodies circulate in the blood, the response they mediate is called the humoral immune response.
Q8. Expand AIDS and name the virus that causes it.
Answer: AIDS stands for Acquired Immuno Deficiency Syndrome. It is caused by the Human Immunodeficiency Virus (HIV), a retrovirus whose RNA genome is enclosed in an envelope. The word "acquired" signals that it is not congenital, and "syndrome" means a group of symptoms.
Q9. What property do normal cells show that cancer cells lose, and what is the most feared property of malignant tumours?
Answer: Normal cells show contact inhibition — contact with neighbouring cells stops their growth. Cancer cells lose this property and divide uncontrollably to form tumours. The most feared property of malignant tumours is metastasis, in which cells break away, travel through the blood and start fresh tumours at distant sites.
Q10. Name the plant source of (a) heroin and (b) cocaine.
Answer: (a) Heroin (smack) is obtained by acetylation of morphine, which is extracted from the latex of the opium poppy Papaver somniferum. (b) Cocaine is obtained from the coca plant Erythroxylum coca, native to South America. Heroin is a depressant; cocaine is a stimulant that interferes with dopamine transport.
Application & Scenarios (Q11–Q20)
Q11. A patient has recurring bouts of chill and high fever roughly every three to four days. Which disease is suggested, and what causes this periodic pattern?
Answer: The pattern points to malaria, caused by Plasmodium. The parasite multiplies inside red blood cells and ruptures them in synchronised waves; each rupture releases a toxic substance called haemozoin, and it is this release that triggers the recurring chill and high fever. The most serious, sometimes fatal form is malignant malaria caused by Plasmodium falciparum.
Q12. Trace how Plasmodium completes its life cycle, naming both hosts and the infective stage that enters the human body.
Answer: Plasmodium needs two hosts — humans and the female Anopheles mosquito. The infective stage, the sporozoite, enters the human body through the bite of an infected female Anopheles. It first multiplies in the liver cells, then attacks and ruptures red blood cells. When a mosquito bites an infected person, the parasites enter the mosquito, develop further, form sporozoites, and collect in its salivary glands ready to infect the next person. The female Anopheles is therefore the vector.
Q13. For a mosquito-borne disease such as malaria or filariasis, list the most effective control measures.
Answer: Because the pathogen spreads through an insect vector, the priority is to control or eliminate the mosquito and its breeding places: avoid stagnant water around homes, clean coolers regularly, use mosquito nets, add larva-eating fish such as Gambusia to ponds, spray insecticides in ditches and swamps, and fit doors and windows with wire mesh. Vector control matters more here than avoiding the patient, since the disease is not passed by direct contact.
Q14. A newborn infant receives protection through the mother's first milk. Identify the type of immunity and the antibody involved.
Answer: This is passive immunity, because ready-made antibodies are supplied rather than made by the infant. The yellowish first milk, colostrum, is rich in IgA antibodies that protect the newborn. The foetus also receives some antibodies from the mother across the placenta during pregnancy. Passive immunity acts quickly but does not last, since the body has not made the antibodies itself.
Q15. A person is bitten by a snake and is given an injection of antivenom. Explain why this is passive immunisation and why active immunity would not help here.
Answer: The antivenom contains preformed antibodies against the venom, given directly to the patient, so this is passive immunisation. It works immediately, which is essential because the venom acts fast. Active immunity would be useless in an emergency because it is slow — the body would need days to make its own antibodies, by which time the venom would already have caused harm. The same logic applies to tetanus antitoxin.
Q16. Vaccination protects against a future infection. Which property of the immune system makes this possible, and what does a vaccine actually contain?
Answer: Vaccination relies on the memory of the immune system. A vaccine contains antigenic proteins of a pathogen, or an inactivated or weakened pathogen. The body responds by making antibodies and, more importantly, memory B-cells and T-cells. On a later real infection these memory cells recognise the pathogen at once and mount a rapid, massive antibody response that overwhelms it before disease develops.
Q17. An organ transplant is rejected by the recipient even after careful surgery. Which arm of the immune system is responsible, and what precautions reduce rejection?
Answer: Graft rejection is driven by the cell-mediated immune response, run by T-lymphocytes, which distinguish 'self' from 'non-self'. To reduce rejection, doctors perform tissue matching and blood-group matching before the transplant, and the patient must take immunosuppressants for life to keep the T-cell response in check.
Q18. Explain, step by step, how HIV brings about a deficiency of the immune system after entering the body.
Answer: After entering the body, HIV first enters macrophages, where its RNA genome is copied into viral DNA by the enzyme reverse transcriptase. This viral DNA integrates into the host cell's DNA and directs the cell to make new virus particles, turning the macrophage into a virus factory. HIV also enters helper T-lymphocytes, replicates there, and the progeny viruses attack still more helper T-cells. The steady fall in helper T-cell numbers cripples defence, so the patient succumbs to infections a healthy person would easily overcome. AIDS is diagnosed using the ELISA test.
Q19. Distinguish a benign tumour from a malignant tumour using their behaviour in the body.
Answer: A benign tumour stays confined to its original site, does not spread and causes little damage. A malignant tumour is a mass of actively dividing neoplastic cells that invade and damage surrounding tissue, compete with normal cells for nutrients, and — most dangerously — undergo metastasis, spreading through the blood to start new tumours elsewhere. This spreading behaviour is what makes malignant tumours life-threatening.
Q20. A heavy smoker is warned about lung cancer. Explain the link between tobacco smoke and cancer, and name the general term for such agents.
Answer: Tobacco smoke contains chemical carcinogens that have been identified as a major cause of lung cancer. Agents that transform normal cells into cancerous ones — whether physical, chemical or biological — are called carcinogens. Someone known to be susceptible to lung cancer is best advised to avoid the specific carcinogen, in this case tobacco smoke. Smoking also raises the blood's carbon monoxide level, cutting oxygen delivery.
Analytical & Multi-Concept (Q21–Q30)
Q21. Two diseases, typhoid and pneumonia, are both bacterial, yet their prevention advice differs. Explain why, linking each to its mode of transmission.
Answer: Typhoid spreads mainly through contaminated food and water, so prevention centres on water and food hygiene — safe drinking water, proper waste disposal and clean food handling. Pneumonia is air-borne, spread by droplets from coughs and sneezes or by sharing utensils, so besides general hygiene the key is to avoid close contact with an infected person and their belongings. The prevention follows directly from how the pathogen travels.
Q22. A child is exposed to a pathogen for the first time and shows a slow, mild reaction; on a second exposure the response is fast and strong. Name the two responses and explain what accounts for the difference.
Answer: The first, low-intensity reaction is the primary response; the faster, more intense second reaction is the secondary (anamnestic) response. The difference is due to immunological memory — the first encounter produces memory B-cells and T-cells that persist. On re-exposure these memory cells recognise the same pathogen at once and generate antibodies far more quickly and in much greater quantity. This memory is the whole basis of vaccination.
Q23. Classify each as innate or acquired immunity, giving a one-line reason: (a) skin as a barrier, (b) interferons from a virus-infected cell, (c) antibodies made after a measles infection, (d) T-cells rejecting a graft.
Answer: (a) Skin — innate (a physical barrier present from birth). (b) Interferons — innate (a cytokine barrier; virus-infected cells secrete them to protect neighbours). (c) Antibodies after measles — acquired (pathogen-specific, made after exposure; humoral). (d) T-cells rejecting a graft — acquired (specific, cell-mediated, distinguishing self from non-self). Innate defences are non-specific; acquired defences are specific and show memory.
Q24. A person suddenly starts sneezing, with watery eyes and a running nose, on entering a dusty room, and the symptoms vanish on leaving. Name the condition, the antibody class involved, and the chemicals responsible for the symptoms.
Answer: This is an allergy — an exaggerated response of the immune system to a harmless environmental antigen (here, dust mites), called an allergen. The antibodies produced against allergens are of the IgE class. The symptoms are caused by the release of histamine and serotonin from mast cells, which is why drugs such as anti-histamines quickly relieve them.
Q25. Rheumatoid arthritis is described as an auto-immune disease. Explain what has gone wrong with the normal working of the immune system.
Answer: Normally the immune system distinguishes 'self' from 'non-self' and attacks only foreign molecules and organisms. In an auto-immune disease this recognition fails and, for genetic and other largely unknown reasons, the body attacks its own cells, causing damage. Rheumatoid arthritis is such a disease, in which self-directed immune attack damages the body's own tissues.
Q26. Both a snakebite victim and a person being vaccinated receive an injection, yet only one develops lasting protection. Compare the two in terms of type of immunity, speed and durability.
Answer: The snakebite victim receives preformed antibodies (antivenom) — this is passive immunity: it acts immediately but is short-lived, because the body itself makes nothing and forms no memory. The vaccinated person receives antigens, prompting the body to make its own antibodies and memory cells — this is active immunity: it develops slowly but is long-lasting. Only active immunity, through memory cells, gives durable protection against future infection.
Q27. Explain why HIV is classified as a retrovirus and why treatment with anti-retroviral drugs can prolong life but cannot cure AIDS.
Answer: HIV is a retrovirus because it carries an RNA genome that is reverse-transcribed into DNA by the enzyme reverse transcriptase — the reverse of the usual DNA-to-RNA flow. That viral DNA then integrates into the host cell's own DNA, so the infection becomes a permanent part of the infected cells. Anti-retroviral drugs can slow viral replication and prolong life, but because the integrated viral DNA cannot be removed, they are only partially effective and cannot prevent the eventual outcome. This is why prevention is stressed as the best option.
Q28. A tumour is found in an internal organ. Outline how it would be detected and diagnosed, and describe the usual combined approach to treatment.
Answer: Detection uses biopsy and histopathological study — a thin section of the suspected tissue is stained and examined under a microscope — supported by imaging such as radiography, CT and MRI for internal organs, and blood and bone-marrow counts in the case of leukaemias. Antibodies against cancer-specific antigens and gene tests for inherited susceptibility also help. Treatment usually combines surgery, radiotherapy and chemotherapy; in addition, biological response modifiers such as alpha-interferon are given to activate the patient's own immune system against the tumour.
Q29. A person who injects drugs intravenously is warned of a higher risk of two serious infections. Name them, explain the shared route of transmission, and connect this to another route by which the same infections spread.
Answer: The two infections are AIDS (HIV) and Hepatitis B. Both spread among intravenous drug users through the sharing of infected needles and syringes, which passes infected blood from one person to another. The same viruses also spread through sexual contact and transfusion of infected blood, and from an infected mother to her child. Both are chronic and ultimately fatal, which is why disposable needles, safe blood and safe practices are emphasised in prevention.
Q30. A teenager begins with occasional drug use out of curiosity and gradually cannot stop, needing larger doses and suffering when the drug is withdrawn. Explain the roles of tolerance, addiction and dependence in this progression, and state one prevention measure.
Answer: With repeated use, the tolerance of the body's receptors rises, so they respond only to higher doses — driving greater intake. Addiction is the psychological attachment to the drug's effects, such as euphoria, that pushes the person to keep taking it even when it is harmful. Dependence is the body's need for the drug, shown by an unpleasant withdrawal syndrome — anxiety, shakiness, nausea, sweating — when the dose is stopped. Together these pull the user into a vicious circle. A sound prevention measure is education and counselling, along with avoiding undue peer pressure, seeking help from parents and teachers, and, where needed, professional de-addiction help.
End of Section 14
You have now worked through 30 examples spanning the common human diseases and their pathogens, the malaria life cycle and vector control, innate and acquired immunity, active versus passive immunity, allergy and auto-immunity, vaccination, the mechanism of HIV/AIDS, cancer biology and detection, and drug and alcohol abuse. For every disease question, remember the three-part habit — pathogen, transmission, prevention — and you will rarely leave marks on the table.