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.

UPSC Prelims · Mains GS-III Vaccines · AMR ~40 min read Disease–Pathogen Pairs Very High CA Weight

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.

Key facts: WBCs are the defence cells; the immune "memory" is why we usually get diseases like measles only once. HIV attacks helper T-cells, crippling immunity (leading to AIDS).

2. Vaccines & Immunisation

A vaccine trains the immune system to recognise a pathogen without causing the disease — giving active immunity in advance.

Fig 5.1 — A vaccine introduces a harmless form of the antigen; the body makes antibodies & memory cells, so a later real infection is defeated quickly.

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.

GS-II hook: India's Universal Immunisation Programme, Mission Indradhanush, and indigenous vaccines (COVAXIN, ROTAVAC, cervical-cancer HPV vaccine) all apply this section. Cold chain keeps vaccines effective.

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

Prelims traps: TB, cholera, typhoid, leprosy, tetanus = bacterial; malaria & kala-azar = protozoan; dengue, polio, AIDS = viral. Antibiotics work on bacteria, NOT viruses — a very common misconception UPSC tests.

Spread modes: air (TB, cold), water/food (cholera, typhoid), contact (leprosy, STDs), vector (malaria, dengue). Understanding the route explains the prevention.

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

Prelims facts: only the female Anopheles spreads malaria; Aedes (day-biting, "tiger" mosquito) spreads dengue, chikungunya & Zika; the same vector can carry several diseases. Control = source reduction, nets, spraying, and larvivorous fish.

GS-III hook: India's National Vector Borne Disease Control Programme and malaria-elimination target (2030) rest on this — climate change is expanding vector ranges.

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.

GS-II hook: the "double burden" — India still fights infectious diseases while NCDs surge. National NCD programme, tobacco control (COTPA), and screening at Ayushman Bharat health centres respond to this.

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".

Response — "One Health": humans, animals & environment tackled together. India's National Action Plan on AMR, Red-Line campaign (Rx-only antibiotics), antibiotic stewardship & new-drug R&D. A classic GS-III + GS-II Mains theme.

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.

Prelims facts: a pandemic is wider than an epidemic; endemic ≠ epidemic; most emerging diseases are zoonotic — hence surveillance of animal–human interfaces.

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

GS-II governance hook: India's health challenges — low public spending, urban-rural gap, workforce shortage — are recurring Mains themes; strengthening primary care & digital health (Ayushman Bharat Digital Mission) is the policy direction.

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.

Link: these tie biology to environment (Topic in Environment subject) and governance — a strong cross-cutting Mains angle on preparedness & One Health.

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

Indigenous vaccines: India's HPV (cervical-cancer) vaccine, TB-vaccine trials, and dengue-vaccine development keep immunisation topical — direct applications of Section 2. check for latest update or data

AMR & One Health: WHO & India flag rising drug resistance; the National Action Plan on AMR and antibiotic-stewardship steps are recurring exam material. check for latest update or data

Outbreaks & surveillance: Nipah flare-ups, dengue surges, mpox and periodic bird-flu alerts stress genomic surveillance & preparedness. 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.

UPSC Prelims — Pathogen

Q: Malaria is caused by which type of organism?

(a) Bacterium
(b) Virus
(c) Protozoan
(d) Fungus

Answer: (c) Malaria is caused by the protozoan Plasmodium, transmitted by the female Anopheles mosquito.

UPSC Prelims — Antibiotics

Q: Antibiotics are effective against —

(a) bacterial infections
(b) viral infections
(c) all infections
(d) genetic disorders

Answer: (a) Antibiotics act on bacteria; they do NOT work against viruses — misuse against viral infections fuels AMR.

UPSC Prelims — Immunity

Q: The immunity a newborn gets from its mother's milk is an example of —

(a) active immunity
(b) passive immunity
(c) innate immunity
(d) herd immunity

Answer: (b) Ready-made antibodies transferred via mother's milk give immediate but short-lived passive immunity.

UPSC Prelims — Vector

Q: Dengue and chikungunya are transmitted by which mosquito?

(a) Anopheles
(b) Culex
(c) Aedes aegypti
(d) Sandfly

Answer: (c) The day-biting Aedes aegypti transmits dengue, chikungunya and Zika; Anopheles spreads malaria.

UPSC Prelims — Bacterial disease

Q: Which of the following is a bacterial disease?

(a) Dengue
(b) Tuberculosis
(c) AIDS
(d) Polio

Answer: (b) TB is caused by the bacterium Mycobacterium tuberculosis; dengue, AIDS and polio are viral.

UPSC Prelims — Immune cells

Q: Antibodies in the human body are produced by —

(a) red blood cells
(b) platelets
(c) B-lymphocytes (WBCs)
(d) plasma

Answer: (c) Antibodies are proteins produced by B-lymphocytes (a type of white blood cell) in the adaptive immune response.

UPSC Prelims — Epidemiology

Q: A disease that spreads across several countries and continents is termed —

(a) endemic
(b) epidemic
(c) pandemic
(d) sporadic

Answer: (c) A pandemic spreads across countries/continents (e.g. COVID-19); an epidemic is regional; endemic means constantly present in an area.

UPSC Prelims — AMR

Q: The term "superbug" refers to —

(a) a large insect vector
(b) a microbe resistant to multiple drugs
(c) a genetically modified crop pest
(d) a beneficial gut bacterium

Answer: (b) A superbug is a microbe (often bacteria) resistant to multiple antimicrobials — the outcome of AMR.

Prelims — anticipated themes

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.

Mains GS-III 15 marks · 250 words

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.

Mains GS-II 15 marks · 250 words

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.

Mains GS-III 10 marks · 150 words

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.

Mains GS-II 15 marks · 250 words

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.

Mains GS-III — anticipated themes

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

Immunity & vaccines:

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

Disease → pathogen:

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

Vectors:

Malaria = Plasmodium, female Anopheles
Dengue/chikungunya/Zika = Aedes aegypti
Filariasis = Culex; kala-azar = sandfly; plague = rat flea

AMR & epidemiology:

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

Highest-frequency themes: disease–pathogen–vector pairs · active vs passive immunity · antibiotics don't kill viruses · vaccine types · AMR & One Health · epidemic vs pandemic · zoonoses · double burden.

Vector	Disease
Anopheles	Malaria
Aedes	Dengue/Zika
Culex	Filariasis
Sandfly	Kala-azar

Type	Key
Active	Own antibodies
Passive	Ready-made
Innate	Born with
Adaptive	Acquired, memory

Topic 5: Immunity, Diseases & Health

On this page

Conceptual Clarity — How UPSC Tests Immunity & Disease

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 (2024–2026)

11. Prelims PYQs

12. Mains PYQs + Model Answers

Model Answer

Model Answer

Model Answer

Model Answer

15-Minute Revision Box

Must-Remember Facts — Immunity, Diseases & Health