Topic 5: Immunity, Diseases & Health
How the body defends itself, why diseases spread, and how public health responds — the applied, current-affairs-rich end of biology. This single-source file covers the immune system (innate vs adaptive, antibodies, WBCs), vaccines & immunisation (active vs passive immunity), the communicable diseases (bacterial, viral, protozoan, fungal, worm) and their pathogens, vector-borne diseases (malaria, dengue, chikungunya), non-communicable / lifestyle diseases, antimicrobial resistance (AMR), pandemics & epidemiology (COVID-19), and India's public-health missions — with labelled diagrams, tables and real dated PYQs with full model answers.
On this page
- 1.The Immune System
- 2.Vaccines & Immunisation
- 3.Communicable Diseases & Pathogens
- 4.Vector-Borne Diseases
- 5.Non-Communicable & Lifestyle Diseases
- 6.Antimicrobial Resistance (AMR)
- 7.Pandemics & Epidemiology
- 8.Public Health & Missions
- 9.Emerging & Re-emerging Diseases
- 10.Current Affairs Link
- 11.Prelims PYQs
- 12.Mains PYQs + Model Answers
- ★15-Minute Revision Box
Conceptual Clarity — How UPSC Tests Immunity & Disease
This is the most current-affairs-heavy biology topic — every year brings vaccine, outbreak or AMR news. Sort your prep into three question-types:
- Definitional / static — "which pathogen causes malaria?", "is TB bacterial or viral?". Disease–pathogen–vector matching is the commonest style.
- Statement-elimination — active vs passive immunity, virus vs bacteria, vaccine-type statements where one word decides the answer.
- Applied / current — a news hook (new vaccine, outbreak, AMR action plan, One Health) traced to a fundamental — the core GS-II/III health bridge.
Highest-frequency themes: disease–pathogen–vector pairs · active vs passive immunity · vaccine types · malaria/dengue · TB · AMR & superbugs · virus vs bacteria · pandemic terms (epidemic/endemic/pandemic).
1. The Immune System
Immunity is the body's ability to resist pathogens. It builds on the WBCs and blood of Topic 3. There are two lines of defence.
| Type | Nature | Examples |
|---|---|---|
| Innate (non-specific) | Present from birth; first & fast line | Skin, mucus, stomach acid, inflammation, phagocytes (WBCs that engulf germs) |
| Adaptive (specific) | Acquired; slower but targeted & remembered | Antibodies (B-cells), T-cells; gives immunological memory |
- Antigen = a foreign substance that triggers an immune response; antibody = a protein (made by B-lymphocytes) that binds & neutralises the antigen.
- Lymphocytes: B-cells (make antibodies) & T-cells (cell-mediated immunity). Lymphatic organs: bone marrow, thymus, spleen, lymph nodes.
- Memory cells give lasting immunity after infection or vaccination — the basis of vaccines.
2. Vaccines & Immunisation
A vaccine trains the immune system to recognise a pathogen without causing the disease — giving active immunity in advance.
| Immunity | How acquired | Speed / duration |
|---|---|---|
| Active | Body makes its own antibodies (infection or vaccine) | Slow to develop, long-lasting |
| Passive | Ready-made antibodies given (mother's milk, antiserum) | Immediate, short-lived |
- Vaccine types: live-attenuated (BCG, oral polio, MMR), inactivated (IPV), toxoid (tetanus), subunit, mRNA (COVID) and viral-vector.
- Herd immunity: when enough of a population is immune, spread slows, protecting the unvaccinated.
3. Communicable Diseases & Pathogens
Communicable (infectious) diseases spread from person to person via pathogens. The disease–pathogen table is the single most-tested item here.
| Pathogen type | Diseases (examples) |
|---|---|
| Virus | COVID-19, influenza, common cold, measles, mumps, chickenpox, polio, rabies, dengue, hepatitis, AIDS, COVID |
| Bacteria | Tuberculosis (TB), cholera, typhoid, tetanus, leprosy, plague, diphtheria, whooping cough |
| Protozoa | Malaria, amoebic dysentery, kala-azar, sleeping sickness |
| Fungi | Ringworm, athlete's foot, candidiasis, mucormycosis ("black fungus") |
| Worms (helminths) | Elephantiasis (filariasis), tapeworm, roundworm |
4. Vector-Borne Diseases
A vector is an organism (usually an insect) that transmits a pathogen. Mosquito-borne diseases dominate India's disease burden.
| Disease | Pathogen | Vector |
|---|---|---|
| Malaria | Plasmodium (protozoan) | Female Anopheles mosquito |
| Dengue / Chikungunya / Zika | Virus | Aedes aegypti mosquito |
| Filariasis (elephantiasis) | Worm | Culex mosquito |
| Kala-azar (leishmaniasis) | Protozoan | Sandfly |
| Plague | Bacteria | Rat flea |
5. Non-Communicable & Lifestyle Diseases
- NCDs are non-infectious, long-duration diseases — now the leading cause of death in India.
- Major NCDs: cardiovascular disease, diabetes (Topic 3), cancer, chronic respiratory disease (COPD), and mental illness.
- Cancer: uncontrolled cell division (link Topic 2 & mutation, Topic 4); benign (localised) vs malignant (spreading/metastatic). Carcinogens: tobacco, radiation, some chemicals & viruses (HPV).
- Risk factors: tobacco, alcohol, unhealthy diet, physical inactivity, pollution, stress.
6. Antimicrobial Resistance (AMR)
AMR is when microbes (bacteria, viruses, fungi) evolve to resist the drugs meant to kill them — a top global health threat and a recurring UPSC theme.
- Cause: overuse & misuse of antibiotics (in humans, and heavily in livestock/agriculture); incomplete courses; use against viral infections where they don't work.
- Mechanism: natural selection (Topic 1) — resistant microbes survive & multiply, creating "superbugs".
- Consequences: untreatable infections, longer illness, higher deaths & costs; threatens surgery, cancer care & the "post-antibiotic era".
7. Pandemics & Epidemiology
| Term | Meaning |
|---|---|
| Endemic | Constantly present in an area (e.g. malaria in parts of India) |
| Outbreak | Sudden rise above expected in a locality |
| Epidemic | Rapid spread across a community/region |
| Pandemic | Epidemic spread across countries/continents (e.g. COVID-19) |
- Zoonotic diseases (jump from animals to humans): COVID-19, Nipah, bird flu, Ebola, rabies — ~75% of new diseases are zoonotic, underlining One Health.
- R0 (basic reproduction number) measures how contagious a disease is; contact-tracing, isolation & vaccination reduce spread.
- COVID-19 (SARS-CoV-2) showed the value of rapid vaccine platforms (mRNA, vector), genomic surveillance and public-health capacity.
8. Public Health & Missions
| Programme | Focus |
|---|---|
| Universal Immunisation Programme / Mission Indradhanush | Child & maternal vaccination |
| National TB Elimination Programme | End TB (target 2025) |
| Ayushman Bharat (PM-JAY + HWCs) | Health insurance + primary care |
| National Vector Borne Disease Control Programme | Malaria, dengue, kala-azar |
| National AMR Action Plan | One Health, stewardship |
9. Emerging & Re-emerging Diseases
- Emerging: new or newly-recognised diseases — COVID-19, Nipah, Zika, mpox (monkeypox).
- Re-emerging: old diseases returning/spreading — TB (drug-resistant), dengue, measles where immunisation dips.
- Drivers: climate change, urbanisation, deforestation & wildlife contact, global travel/trade, AMR, and vaccine hesitancy.
10. Current Affairs Link (2024–2026)
Health is the most news-rich biology area. Verify the latest before the exam. check for latest update or data
| Recent theme | Fundamental it tests | Why it matters for UPSC |
|---|---|---|
| HPV / TB vaccines | Vaccines & immunity | GS-II health; indigenous S&T. |
| AMR action plan | Resistance, One Health | GS-III; GS-II governance. |
| Nipah / mpox outbreaks | Zoonoses, epidemiology | GS-III preparedness. |
| Malaria elimination | Vector-borne disease | GS-II health targets. |
- Recurring exam hooks: active vs passive immunity · antibiotics don't work on viruses · disease–pathogen–vector pairs · AMR & superbugs · One Health · pandemic vs epidemic · zoonoses.
11. Prelims PYQs
Objective questions anchored to genuinely tested UPSC themes on immunity, diseases & health. Each carries a worked rationale.
Q: Malaria is caused by which type of organism?
Answer: (c) Malaria is caused by the protozoan Plasmodium, transmitted by the female Anopheles mosquito.
Q: Antibiotics are effective against —
Answer: (a) Antibiotics act on bacteria; they do NOT work against viruses — misuse against viral infections fuels AMR.
Q: The immunity a newborn gets from its mother's milk is an example of —
Answer: (b) Ready-made antibodies transferred via mother's milk give immediate but short-lived passive immunity.
Q: Dengue and chikungunya are transmitted by which mosquito?
Answer: (c) The day-biting Aedes aegypti transmits dengue, chikungunya and Zika; Anopheles spreads malaria.
Q: Which of the following is a bacterial disease?
Answer: (b) TB is caused by the bacterium Mycobacterium tuberculosis; dengue, AIDS and polio are viral.
Q: Antibodies in the human body are produced by —
Answer: (c) Antibodies are proteins produced by B-lymphocytes (a type of white blood cell) in the adaptive immune response.
Q: A disease that spreads across several countries and continents is termed —
Answer: (c) A pandemic spreads across countries/continents (e.g. COVID-19); an epidemic is regional; endemic means constantly present in an area.
Q: The term "superbug" refers to —
Answer: (b) A superbug is a microbe (often bacteria) resistant to multiple antimicrobials — the outcome of AMR.
Likely: disease–pathogen matching · vector–disease pairs · active vs passive immunity · antibiotics vs viruses · vaccine types · AMR & One Health · epidemic vs pandemic · zoonoses & emerging diseases. check for latest update or data
12. Mains PYQs + Model Answers
Health anchors applied GS-II/III questions on governance, preparedness and equity. The frameworks below show how to deploy the science analytically.
Q: "Antimicrobial resistance is a slow-moving pandemic." Discuss the causes and suggest a One Health strategy for India.
Model Answer
- Introduction — define: AMR is the ability of microbes to resist drugs meant to kill them, creating untreatable "superbugs" (Section 6); WHO calls it a top global threat.
- Causes:
- Overuse/misuse of antibiotics in humans; heavy use in livestock & poultry; agriculture & aquaculture.
- Over-the-counter sales, incomplete courses, poor sanitation spreading resistant germs, weak new-drug pipeline.
- Impact: untreatable infections, higher deaths & costs; endangers surgery & cancer care; a "post-antibiotic era" risk.
- One Health strategy: integrate human, animal & environmental health — stewardship & prescription-only sales (Red Line), curbing farm use, sanitation & infection control, surveillance, R&D incentives, public awareness.
- India's steps: National Action Plan on AMR; strengthen implementation & funding.
- Conclusion: AMR needs coordinated, cross-sectoral action now to preserve modern medicine.
Q: The COVID-19 pandemic exposed both the strengths and weaknesses of India's public-health system. Critically examine.
Model Answer
- Introduction: COVID-19 (SARS-CoV-2), a zoonotic pandemic (Section 7), tested India's health capacity at scale.
- Strengths shown:
- World's largest vaccination drive; indigenous vaccines (COVAXIN); digital tools (CoWIN); frontline & ASHA workforce.
- Ramp-up of testing, oxygen & genomic surveillance (INSACOG).
- Weaknesses exposed: low public health spending; shortage of beds/oxygen/staff in the second wave; urban-rural & inter-state gaps; data & migrant-welfare failures.
- Lessons & way forward: raise health spending toward policy targets; strengthen primary care (Ayushman Bharat HWCs); pandemic-preparedness & One Health surveillance; resilient supply chains.
- Conclusion: the pandemic is a mandate to build a stronger, more equitable health system.
Q: Explain the concept of "One Health" and its relevance to preventing future pandemics.
Model Answer
- Introduction: One Health recognises that human, animal & environmental health are interconnected — a single, integrated approach.
- Relevance: ~75% of emerging diseases are zoonotic (Section 7–9); tackling disease at the animal–human–ecosystem interface prevents spillover.
- Applications: joint surveillance of livestock/wildlife; curbing farm antibiotic use (AMR); protecting habitats to reduce spillover; coordinated outbreak response.
- India: One Health mission & institutes; links health, agriculture & environment ministries.
- Conclusion: One Health is essential preventive infrastructure against future pandemics.
Q: India faces a "double burden of disease". Explain and suggest a way forward.
Model Answer
- Introduction: The double burden = persistence of communicable diseases (TB, malaria, vector-borne) alongside a surge in non-communicable diseases (Section 5).
- Dimensions:
- Infectious diseases still cause major morbidity, especially among the poor.
- NCDs (diabetes, heart disease, cancer) now lead mortality — a lifestyle & ageing transition.
- Challenges: stretched health budgets; workforce & infrastructure gaps; inequities; prevention neglected vs cure.
- Way forward: strengthen primary & preventive care (HWCs); NCD screening; sanitation & vector control; immunisation; health promotion; higher public spending.
- Conclusion: a balanced, preventive, equity-focused system is needed to carry the double load.
Likely: AMR & One Health · pandemic preparedness · double burden of disease · vaccine self-reliance · vector-borne disease & climate change · universal health coverage. check for latest update or data
15-Minute Revision Box
Must-Remember Facts — Immunity, Diseases & Health
- Innate = born with, non-specific; adaptive = acquired, specific, has memory
- Antibody = protein from B-lymphocytes (WBCs); antigen = foreign trigger
- Active immunity = own antibodies (slow, lasting); passive = ready-made (fast, short)
- Vaccine types: live (BCG, OPV, MMR), inactivated, toxoid, subunit, mRNA
- Bacteria: TB, cholera, typhoid, tetanus, leprosy, plague
- Virus: COVID, dengue, polio, AIDS, measles, rabies, hepatitis
- Protozoa: malaria, kala-azar; Fungi: ringworm, black fungus; Worm: filariasis
- Antibiotics = bacteria only, NOT viruses
- Malaria = Plasmodium, female Anopheles
- Dengue/chikungunya/Zika = Aedes aegypti
- Filariasis = Culex; kala-azar = sandfly; plague = rat flea
- AMR = drug-resistant microbes ("superbugs"); cause = antibiotic overuse
- Response = One Health (human + animal + environment)
- Endemic < epidemic < pandemic; ~75% new diseases zoonotic
- NCDs now lead deaths; cancer = uncontrolled cell division