Diseases & Immunity – NDA Biology Notes

The human body is constantly exposed to threats. Bacteria, viruses, and parasites try to invade. The body fights back. When the body loses that fight, or when something inside the body goes wrong, disease results.

A disease is any condition that disrupts normal body function. Diseases can be caused by pathogens (organisms that cause disease), genetic defects, nutritional deficiencies, or environmental damage. Understanding diseases in this chapter requires knowing three things for each disease: what causes it, how it spreads, and which specific facts the exam targets.

For the NDA exam, this chapter is one of the highest-frequency sections in all of biology. Questions come from disease classification, the vector map, the bile-versus-antibiotic confusion for viruses, HIV structure, malaria pathology, dengue mechanisms, and diagnostic tests. Every section below marks exactly where exam questions originate.

1. Master Disease Classification

This table is the foundation of the chapter. Every disease listed here has been tested by NDA. Know every entry.

Causative Agent	Category	Diseases
Bacteria	Bacterial	Typhoid, Tuberculosis, Cholera, Leprosy, Tetanus, Diphtheria, Pertussis, Plague, Anthrax, Bacillary Dysentery
Virus	Viral	AIDS, Dengue, Polio, Measles, Chickenpox, Rabies, Smallpox, Influenza, Hepatitis B, Chikungunya, Bird flu, Swine flu, COVID-19
Protozoan	Protozoal	Malaria (Plasmodium), Kala-azar (Leishmania), Sleeping Sickness (Trypanosoma), Amoebic Dysentery
Worm (Helminth)	Helminthic	Elephantiasis / Filariasis (Wuchereria bancrofti), Ascariasis (Ascaris lumbricoides)
Genetic defect	Genetic	Haemophilia, Colour Blindness, Down’s Syndrome, Sickle-cell Anaemia
Vitamin / Mineral deficiency	Nutritional	Xerophthalmia (Vit A), Pellagra (Vit B3), Rickets (Vit D), Goitre (Iodine)
Occupation / Environment	Environmental	Silicosis, Asbestosis, Skin cancer (UV radiation)

2. Bacterial Diseases

Bacterial diseases are the most misclassified category in the chapter. Three diseases are most frequently and incorrectly called viral in exam options.

★ IMPORTANT Cholera is bacterial: caused by Vibrio cholerae. NOT viral.  [NDA 2007-II | NDA 2019-I] Tetanus is bacterial: caused by Clostridium tetani. When tetanus appears alongside chickenpox, measles, and polio in a “which is NOT viral?” question, tetanus is the answer.  [NDA 2008-II] Leprosy is bacterial: caused by Mycobacterium leprae. NOT viral, NOT protozoal.  [NDA 2014-II] Every time an exam lists cholera, tetanus, or leprosy as viral diseases, be cautious when answering the question.

The complete set of bacterial diseases asked in NDA multiple times: [NDA 2007-II | NDA 2008-II | NDA 2013-I | NDA 2019-II | NDA 2022-I]

Disease	Causative Bacterium	Key Facts
Typhoid	Salmonella typhi	Faecal-oral route; houseflies as mechanical carriers [NDA 2007-II | NDA 2013-I | NDA 2022-I]
Tuberculosis	Mycobacterium tuberculosis	Airborne; NOT waterborne  [NDA 2013-I]
Cholera	Vibrio cholerae	Waterborne; NOT viral [NDA 2007-II | NDA 2019-I]
Leprosy	Mycobacterium leprae	NOT viral, NOT protozoal [NDA 2014-II]
Tetanus	Clostridium tetani	NOT viral [NDA 2008-II]
Diphtheria	Corynebacterium diphtheriae	Respiratory transmission
Pertussis (Whooping Cough)	Bordetella pertussis	Childhood respiratory disease
Anthrax	Bacillus anthracis	Potential biological warfare agent
Bacillary Dysentery	Shigella dysenteriae	Waterborne; faecal-oral route
Plague	Yersinia pestis	Haffkine Institute, Mumbai produces plague vaccine [NDA 2009-II]

3. Viral Diseases

Viruses are the smallest pathogens. They cannot reproduce on their own. They are obligate intracellular parasites that can only reproduce inside living host cells. Outside a host, they behave like inert chemical particles. They cannot make food through photosynthesis.  [NDA 2019-II]

RNA viruses mutate faster than DNA viruses. RNA is structurally less stable, and RNA replication lacks the error-checking mechanisms found in DNA replication. HIV is an RNA virus, which is why it mutates rapidly and is difficult to target with fixed drugs.

Key Viral Diseases

Smallpox is caused by the Variola virus.  [NDA 2021-II] It is the only infectious disease to have been officially declared eradicated worldwide. The name Variola is a direct, recurring exam target.

Hepatitis A is waterborne and viral. Hepatitis B is transmitted person-to-person through blood and sexual contact. These are two different diseases with different transmission routes.  [NDA 2007-II | NDA 2012-II]

Dengue is viral: caused by a flavivirus (flavi-ribo virus). The vector is the Aedes mosquito. Any exam option calling dengue non-viral, bacterial, or protozoal is wrong.

4. Protozoan Diseases

Three protozoan diseases are consistently confused with each other in exam options. They must be learned side by side: different organisms, different diseases, different vectors.

Disease	Causative Organism	Vector
Malaria	Plasmodium (protozoan)	Female Anopheles mosquito [NDA 2025-I]
Kala-azar	Leishmania (protozoan)	Sand fly (NOT mosquito) [NDA 2008-I]
Sleeping Sickness	Trypanosoma (protozoan)	Tsetse fly [NDA 2017-I]
Amoebic Dysentery	Entamoeba histolytica	Contaminated food/water

★ IMPORTANT Trypanosoma causes sleeping sickness: vector is the tsetse fly. Leishmania causes kala-azar: vector is the sand fly. These are different organisms causing different diseases through different vectors. They cannot be swapped in an exam answer. Kala-azar is transmitted by the sand fly, NOT the mosquito. When a mosquito appears as the kala-azar vector, it is always wrong. Leishmania multiplies inside the human host by binary fission: asexual reproduction.  [NDA 2010-I]

5. Helminthic Diseases

Elephantiasis (lymphatic filariasis) is caused by the filarial worm Wuchereria bancrofti. The scientific name Wuchereria bancrofti is a direct, recurring exam target. The vector is the Culex mosquito. The disease causes massive swelling of limbs.  [NDA 2017-II]

Ascaris lumbricoides is a roundworm classified under phylum Nematoda.

6. Transmission Routes

Waterborne Diseases

The following diseases spread through contaminated water: Cholera, Typhoid, Amoebic Dysentery, Bacillary Dysentery, Hepatitis A, Jaundice.  [NDA 2018-II]

What is NOT waterborne: Tuberculosis (spreads through air [NDA 2013-III]), Measles (spreads through respiratory droplets [NDA 2007-II]), Rabies (spreads through animal bites), Arthritis (not an infectious disease).  [NDA 2007-II | NDA 2012-II | NDA 2013-III | NDA 2018-II]

Rainwater in the first 30–40 minutes is unsafe. It carries dissolved toxic atmospheric chemicals. The primary concern is chemical toxicity, not bacteria.

Airborne Diseases

Tuberculosis is the primary airborne disease tested in this chapter. It spreads through respiratory droplets when an infected person coughs, sneezes, or speaks. Crowded spaces increase risk significantly.  

Dengue, Malaria, and Cholera are definitively NOT airborne.

Person-to-Person Transmission

AIDS (HIV), Hepatitis B, and Syphilis spread directly between humans through blood and sexual contact.

Cirrhosis is NOT an infectious disease. It is a liver condition caused by alcohol or chronic hepatitis. It does not spread from person to person.

Rabies is transmitted through bites or scratches from infected animals. Dogs are the most common source. Bats are also confirmed reservoirs. This surprises students but is a tested fact.

Swine flu (H1N1) spreads from infected persons, not through eating pork. Consumption of pork does not cause swine flu.  [NDA 2010-II]

7. Vector-Disease Map

This table covers the entire vector-borne disease cluster. It is the highest-frequency exam cluster in the chapter.

Vector	Disease	Causative Organism
Female Anopheles mosquito	Malaria	Plasmodium (protozoan) [NDA 2025-I]
Culex mosquito	Elephantiasis / Filariasis	Wuchereria bancrofti (worm)
Aedes mosquito	Dengue, Yellow Fever, Chikungunya	Flavivirus (virus)
Sand fly	Kala-azar	Leishmania (protozoan) [NDA 2008-I]
Tsetse fly	Sleeping Sickness	Trypanosoma (protozoan)

Disease-transmitting insects are specifically called vectors. The mosquito is the vector of malaria, not the cause. The actual cause is Plasmodium. This is the most commonly confused agent-vector pair in all of UPSC science.

8. Malaria: In Detail

The Organism and Vector

Malaria is caused by Plasmodium, a protozoan. Not a bacterium, not a virus, not the mosquito. The vector is the female Anopheles mosquito, specifically female. Males do not bite.  [NDA 2025-I]

NDA 2025 tested both Plasmodium and Mycobacterium as a deliberate confusion pair. Mycobacterium causes TB: not malaria.  [NDA 2025-I]

Inside the Human Body

After the mosquito bite, Plasmodium sporozoites enter the blood and travel first to the liver. In the liver, they multiply silently. This is the pre-erythrocytic (hepatic) stage. No fever occurs yet.  [NDA 2012-II]

After multiplying in the liver, Plasmodium invades red blood cells. RBCs burst periodically, causing the characteristic cyclic fever of malaria. In severe cases, the brain can be affected. This is cerebral malaria.

The kidney is NOT a primary site of Plasmodium activity.  [NDA 2015-II]

Treatment and Vaccine

No effective malaria vaccine was available for most of the period covered by these PYQs. This absence is a recurring exam point.

Quinine is the original anti-malarial drug. It is derived from the bark of the cinchona tree, not neem, not eucalyptus, not cinnamon.

9. Dengue: In Detail

Dengue is caused by a flavivirus (flavi-ribo virus). The vector is the Aedes mosquito. It is definitively a viral disease.

Dengue is also called “breakbone fever”, because of the severe joint and muscle pain it causes. This name was asked directly.  [NDA 2014-I]

Effect on Blood

Dengue reduces platelet count through three mechanisms: [NDA 2017-I]

• Interfering with platelet production in bone marrow
• Infecting endothelial cells lining blood vessels
• Binding directly with circulating platelets

Accumulation of platelets in the intestine is NOT a mechanism of dengue. It appears as a wrong option in exam questions.

10. Tuberculosis: In Detail

TB is caused by Mycobacterium tuberculosis: a bacterium.  [NDA 2013-I] It spreads through the air: via respiratory droplets released by coughing, sneezing, or speaking. It is NOT waterborne. Crowded, poorly ventilated spaces are high-risk environments.

The DOTS Programme

The WHO strategy for TB control is DOTS, Directly Observed Treatment, Short-Course. Its purpose is ensuring patients complete their full course of drug treatment. Incomplete treatment causes drug-resistant TB.

DOTS does NOT mean a shorter treatment period. It means ensuring the full course is completed under observation.

11. AIDS and HIV: In Detail

AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV: Human Immunodeficiency Virus.

What Kind of Virus Is HIV?

HIV is a retrovirus.  [NDA 2011-II] Its genetic material is single-stranded RNA, not DNA, not double-stranded RNA, not circular DNA.  [NDA 2018-I] Being a retrovirus, HIV uses an enzyme called reverse transcriptase to convert its RNA into DNA inside the host cell.

What HIV Destroys

HIV selectively targets and destroys lymphocytes, specifically CD4+ T-helper lymphocytes. Lymphocytes coordinate the entire immune response. As HIV destroys them, the immune system collapses. The patient becomes vulnerable to infections that a healthy immune system would easily control. HIV does NOT primarily destroy mast cells, platelets, or erythrocytes.

Clinical Signs

Major clinical signs of AIDS in children under 12: loss of weight, chronic diarrhoea, and prolonged fever.  [NDA 2012-II] Persistent cough is associated with TB, not listed as a major paediatric AIDS criterion.

Chronic diarrhoea in a young person is the most strongly associated symptom pointing toward HIV/AIDS.

12. Diagnostic Tests

This cluster is tested in match-the-following format and has very high repeat probability.  [NDA 2009-I]

Disease	Diagnostic Test
AIDS	ELISA Test [NDA 2009-I]
Typhoid	Widal Test [NDA 2009-I]
Tuberculosis	Mantoux (tuberculin skin) test [NDA 2009-I]
Plague	Wayson Stain Test [NDA 2009-I]

This map must be memorised as a complete set. Mixing any two tests is a common error.

13. Immunity

When a foreign pathogen enters the body, the immune system responds. White blood cells identify the threat. Lymphocytes produce specific proteins called antibodies that neutralise the pathogen.

The cells that produce antibodies are lymphocytes, the precise, consistent NDA-level answer.  [NDA 2008-II | NDA 2010-I | NDA 2024-II | NDA 2025-I] Not erythrocytes (which carry oxygen). Not platelets (which clot blood). Not granulocytes, monocytes, or eosinophils. Only lymphocytes produce antibodies.

The organs involved in generating immune responses are: thymus, spleen, lymph nodes, and bone marrow. The brain is NOT an immune organ. It plays no direct role in generating immune responses.  [NDA 2013-II]

Colostrum is the first milk produced by a mother after birth. It is rich in maternal antibodies, especially IgA. It provides the newborn with passive immune protection while their own immune system develops.

14. Edward Jenner and Vaccination

Edward Jenner introduced immunisation to the medical world in 1796. He demonstrated that cowpox infection protected against smallpox, establishing the foundational principle of vaccination. Robert Koch identified disease-causing bacteria. Robert Hooke is associated with the microscope and cell discovery. Carl Linnaeus developed biological classification. None of these three is associated with vaccination.

Haffkine Institute, Mumbai, is specifically associated with plague vaccine production.  [NDA 2009-II]

Category	Diseases
Vaccines exist for	Tetanus, Polio, Pertussis, Measles, Smallpox, Chickenpox, Typhoid, Hepatitis B, Diphtheria, Mumps
No effective vaccine (historically)	Leprosy, Malaria

15. Antibiotics

Three facts about antibiotics appear across six separate exam questions. They must be learned as one connected story.

Who Discovered Penicillin?

Alexander Fleming discovered penicillin in 1928, the first antibiotic.  [NDA 2025-I] The first antibiotic was penicillin, not tetracycline. Fleming, not Crick, not Darwin, not Wilkins.

How Penicillin Kills Bacteria

Penicillin kills bacteria by inhibiting bacterial cell wall synthesis. Without a complete cell wall, bacteria cannot maintain their internal pressure. They burst and die.

Why doesn’t penicillin harm human cells? Human cells have no cell wall, only a plasma membrane. Penicillin targets the cell wall, which human cells simply do not have. So penicillin kills bacteria selectively without harming the human body.

Antibiotics target bacteria through two mechanisms: cell wall synthesis inhibition and bacterial protein synthesis inhibition. They do NOT target nuclear membrane synthesis (bacteria have no nuclear membrane) and NOT mitochondrial function.

Antibiotics Are Completely Ineffective Against Viruses

Antibiotics are completely ineffective against all viral infections.  [NDA 2020-I] Viruses have no cell walls. Viruses do not use bacterial ribosomes. They use the host cell’s own machinery. Antibiotics have nothing to target in a virus. A vaccine can protect against a virus. An antibiotic cannot.

Penicillin Resistance

Resistant bacteria produce an enzyme called β-lactamase (penicillinase). This enzyme chemically degrades the penicillin molecule before it can act.  [NDA 2019-II] Resistant bacteria do NOT store penicillin in vacuoles.

Drug Sources and Special Antibiotics

Drug	Source
Penicillin	Penicillium mould (fungus)
Quinine	Cinchona tree bark
Morphine	Poppy plant
Tetracycline	Streptomyces bacteria

Chloramphenicol is the antibiotic of last resort, reserved for life-threatening infections when all other antibiotics have failed.

Probiotics are live microbial food supplements, NOT antibiotics, NOT antacids. They contain beneficial bacteria contributing to gut health.  [NDA 2017-II]

16. Hereditary Diseases

Hereditary diseases are passed from parent to offspring through genes. They cannot be caught from another person.

Status	Disease	Key Detail
Hereditary	Haemophilia	X-linked; blood fails to clot normally [NDA 2006-II]
Hereditary	Colour Blindness	X-linked recessive; mutation on X chromosome; far more common in males [NDA 2011-I]
Hereditary	Down’s Syndrome	Trisomy 21; extra copy of chromosome 21; mental retardation and early ageing
Hereditary	Sickle-cell Anaemia	Abnormal haemoglobin in RBCs; affects red blood cells only [NDA 2012-II]
NOT hereditary	Cataract	Age-related and UV-related lens clouding
NOT hereditary	Pellagra	Vitamin B3 deficiency
NOT hereditary	Osteoporosis	Bone loss with age
NOT hereditary	Xerophthalmia	Vitamin A deficiency (dry eyes): NOT genetic [NDA 2007-II]

When a question asks “which is NOT a genetic disorder?” among this group, Xerophthalmia is the standard correct answer.

Colour blindness is X-linked recessive. Males carry only one X chromosome. A single mutated copy causes the condition. Females need two mutated copies. This explains why colour blindness is far more common in males.  [NDA 2011-I]

Down’s Syndrome features: mental retardation, early ageing, genetic/chromosomal disorder. A furrowed tongue with open mouth is NOT a standard diagnostic feature.

17. Cancer

In a healthy body, cell growth and division are tightly controlled. In cancer, the regulatory mechanisms controlling cell growth break down. Cells divide uncontrollably and form tumours, either benign (localised) or malignant (invasive).

Cancer is more common in older people because they have had more years to accumulate DNA mutations. The more times cells divide over a lifetime, the more opportunity for mutations to accumulate in cancer-suppressing genes.

Skin cancer is generally caused by ultraviolet (UV) radiation from sunlight. The ozone layer in the stratosphere normally blocks UV radiation from reaching Earth’s surface. When the ozone layer thins, more UV reaches the surface and skin cancer risk increases. The statement “the stratosphere allows UV radiation to enter” is false.

Cigarette smoking exposes the smoker to benzene, a known carcinogen. Both facts, benzene in cigarette smoke and benzene being carcinogenic, are independently correct.

Stem Cell Therapy (SCT) has shown potential for kidney failure, brain injury, and vision impairment. It is NOT an effective treatment for cancer. Cancer involves uncontrolled proliferation, the opposite of the controlled cell replacement that SCT provides.  [NDA 2008-II]

18. Occupational and Special Diseases

Silicosis is an occupational lung disease in carpet weavers and textile workers exposed to silica dust.

Dropsy is caused by adulteration of mustard oil with argemone oil. It causes widespread oedema and cardiovascular complications.  [NDA 2014-II]

Progeria is a rare genetic condition characterised by accelerated ageing. Affected children age several times faster than normal.

Formalin is used as a disinfectant and biological specimen preservative. It is a solution of aqueous formaldehyde, not ferrous sulphate, not formic acid, not ferric alum.

Jaundice is caused by incomplete metabolism of bilirubin, the yellow pigment produced when haemoglobin breaks down. When the liver fails to process bilirubin, it accumulates in the skin and eyes. Jaundice is caused by bilirubin, not biliverdin. Biliverdin is the green precursor that is quickly converted to bilirubin.  [NDA 2009-II]

19. Epidemiological Terminology

Term	Definition	Example
Endemic	A disease constantly maintained at a baseline level in a specific geographic area: no sudden outbreaks	Elephantiasis is endemic: NOT epidemic
Epidemic	Sudden increase in disease cases beyond normal expectation in a defined area	Cholera outbreak in a city
Pandemic	An epidemic that has spread across multiple countries or continents	COVID-19 (2020)

Elephantiasis is endemic, not epidemic. It persists at a steady level in tropical zones and does not cause sudden outbreak events.

Disease-to-Body-Part Mapping

This cluster is tested in match-the-following format.

Disease	Body Part Affected
Conjunctivitis	Eyes
Dermatitis	Skin
Gout	Joints (uric acid crystal deposits)
Meningitis	Spinal cord / meninges

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