NDA Biology · Microbiology and Disease
Pathogens and Diseases — the Master Pairings
A pathogen is a disease-causing organism; the NDA tests you on which exact organism causes which disease, and what KIND of organism it is — virus, bacterium, protozoan, fungus or worm.
Why this matters
This is the heart of the chapter — 13 PYQs, all EASY or MODERATE, all pure recall. The single highest-yield skill is the disease↔pathogen↔type table: knowing malaria is a protozoan (Plasmodium), TB a bacterium (Mycobacterium), AIDS a virus (HIV), and elephantiasis a worm (Wuchereria). The bank's signature trap is the swapped pair (Malaria : Mycobacterium), so learn each pairing in BOTH directions. Beyond the pairings, three side-facts recur: how a disease spreads (waterborne vs airborne), the genetic material of a virus, and the cell a disease attacks (dengue drops platelets).
Concept 1 of 5: The five kinds of pathogen
Every disease-causing organism is one of five kinds: virus, bacterium, protozoan, fungus or worm. Before you can name the exact organism, classify the kind — most NDA questions are won or lost at this first step, because the wrong-kind distractors are the easy eliminations.
Definition
A pathogen is any organism that causes disease. The five kinds, smallest to largest:
- Virus — not truly alive; a protein coat around DNA or RNA; reproduces only inside a host cell. Causes AIDS, dengue, smallpox, COVID-19.
- Bacterium — a single-celled organism with a cell wall but no nucleus (prokaryote). Causes TB, cholera, typhoid, tetanus.
- Protozoan — a single-celled organism WITH a nucleus (eukaryote). Causes malaria, sleeping sickness, amoebic dysentery.
- Fungus — yeasts and moulds; eukaryotic. Causes ringworm, athlete's foot.
- Worm (helminth) — a multicellular parasite. Causes elephantiasis, ascariasis (roundworm).
Match the disease to the KIND of pathogen first — most NDA questions hinge on it.
Worked example
Classify the cause of each disease by kind of pathogen: (a) tuberculosis, (b) malaria, (c) smallpox, (d) elephantiasis.
Practice this concept4 quick reps
Concept 2 of 5: The disease–pathogen–type table
This is the table the whole chapter is built on. Each row pins a disease to the exact organism that causes it and the kind of organism that is. Read it down (disease → cause) and across (cause → kind) until both directions are automatic — the bank tests both.
Definition
The high-yield disease↔pathogen↔type pairings the NDA has tested. Learn the bold organism name with its disease:
- Protozoan diseases: malaria (Plasmodium), sleeping sickness (Trypanosoma), amoebic dysentery (Entamoeba).
- Bacterial diseases: tuberculosis (Mycobacterium), cholera (Vibrio cholerae), typhoid (Salmonella typhi), tooth decay (Streptococcus mutans), tetanus (Clostridium tetani).
- Viral diseases: AIDS (HIV), dengue, smallpox (Variola), COVID-19, chickenpox (Varicella zoster).
- Worm diseases: elephantiasis / filariasis (Wuchereria bancrofti), ascariasis (Ascaris).
| Disease | Pathogen | Type |
|---|---|---|
| Malaria | Plasmodium | Protozoan |
| Sleeping sickness | Trypanosoma | ProtozoanQ Transmitted by the tsetse fly. NDA 2017. |
| Tuberculosis (TB) | Mycobacterium | BacteriumQ TB = Mycobacterium, NOT Plasmodium. The bank swaps this with malaria. NDA 2025. |
| Cholera | Vibrio cholerae | Bacterium |
| Typhoid | Salmonella typhi | BacteriumQ AIDS, dengue and COVID-19 are viral; typhoid is the bacterial one. NDA 2019, 2022. |
| Smallpox | Variola virus | VirusQ Eradicated worldwide in 1980 (WHO). NDA 2021. |
| AIDS | HIV | Virus |
| Chickenpox | Varicella zoster | Virus |
| Elephantiasis (filariasis) | Wuchereria bancrofti | Worm |
The swapped pair is the classic trap: Malaria is Plasmodium (protozoan), TB is Mycobacterium (bacterium) — never the reverse.
Practice this conceptself-check · 5 quick reps
The same idea in a real exam question:
NDA · Apr 2025 · Q114Easy
Example 2 · Microbiology and Disease · Pathogens and Diseases
Which of the following pairs is/are correctly matched?
1. Malaria : Mycobacterium
2. TB : Plasmodium
Select the answer using the code given below:
Malaria ↔ Plasmodium, TB ↔ Mycobacterium — never swapped
The bank's favourite item gives 'Malaria : Mycobacterium' and 'TB : Plasmodium' and asks which is correct — the answer is neither. Malaria is Plasmodium (protozoan); TB is Mycobacterium (bacterium). Lock both pairings in both directions.
Typhoid is the bacterial odd-one-out
When a list mixes AIDS, dengue, COVID-19 and typhoid and asks which is bacterial, the answer is typhoid (Salmonella typhi). The other three are all viral.
Sleeping sickness is a protozoan, not a worm
Sleeping sickness is caused by Trypanosoma, a single-celled protozoan — not a worm. Distractors offer Naegleria, Histomonas; the right answer is Trypanosoma, carried by the tsetse fly.
Concept 3 of 5: How diseases spread — waterborne, airborne, vector
Knowing the cause is half the marks; knowing the route of spread is the other half. The NDA asks 'which one is a waterborne disease?' and 'cholera is caused by ___' — both are route questions. Group the common diseases by how they reach you.
Definition
Diseases grouped by route of transmission:
- Waterborne (contaminated food/water): cholera, typhoid, jaundice (hepatitis A/E), amoebic dysentery. Caused by drinking or eating contaminated material.
- Airborne (droplets): tuberculosis, common cold, influenza, COVID-19.
- Vector-borne (an insect carrier): malaria (Anopheles mosquito), dengue (Aedes mosquito), filariasis (Culex mosquito), sleeping sickness (tsetse fly).
- Animal bite: rabies (dog/bat bite). Non-infectious conditions like arthritis spread by no route at all.
| Route | Diseases | Key fact |
|---|---|---|
| Waterborne | Cholera, typhoid, jaundice | Spread by contaminated food or waterQ Jaundice is the waterborne answer when paired against TB (air), rabies (bite), arthritis (none). NDA 2018. |
| Cholera (route) | Vibrio cholerae | Contaminated food/water → severe watery diarrhoeaQ Not loss of memory, not a muscle disease, not genetic. NDA 2019. |
| Airborne | Tuberculosis, cold, flu, COVID-19 | Spread by respiratory droplets |
| Vector-borne | Malaria, dengue, filariasis | Carried by a mosquito or fly |
| Animal bite | Rabies | Dog or bat bite; not waterborne |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
NDA · Apr 2019 · Q150Easy
Example 3 · Microbiology and Disease · Pathogens and Diseases
Which one of the following statements regarding Cholera is correct?
Cholera is NOT a memory, muscle or genetic disease
Distractors recast cholera as 'loss of memory', a 'muscle disease from alcohol', or a 'genetic disease'. It is a bacterial infection (Vibrio cholerae) from contaminated food or water causing watery diarrhoea.
TB is airborne, not waterborne
When asked for a waterborne disease, tuberculosis is a distractor — it spreads through air. The waterborne answer is usually jaundice, cholera or typhoid.
Concept 4 of 5: Viruses — nature and genetic material
Viruses sit on the border of living and non-living: inert like a chemical outside a host, but able to reproduce inside one. The NDA tests two things — what viruses can and cannot do, and the genetic material of specific viruses (AIDS is the favourite).
Definition
What the bank tests about viruses:
- A virus is a protein coat around genetic material (DNA or RNA, never both); it has no cell, no organelles, no metabolism of its own.
- Viruses need a living host cell to reproduce — they cannot multiply on their own.
- Outside a host they behave like inert chemical particles and can even be crystallised.
- They cannot make their own food — no photosynthesis, no respiration (they lack the organelles).
- HIV (AIDS) is a retrovirus whose genetic material is single-stranded RNA.
| Statement about viruses | True or false |
|---|---|
| Need living cells to reproduce | True |
| All viruses are parasites | True |
| Can synthesize food by photosynthesis | FalseQ Viruses have no chloroplasts or organelles — they cannot photosynthesise. The 'NOT true' answer. NDA 2019. |
| Behave like chemicals outside a host | True |
| HIV genetic material | Single-stranded RNAQ HIV is a retrovirus — single-stranded RNA, not DNA. NDA 2018. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
NDA · Apr 2018 · Q69Moderate
Example 4 · Microbiology and Disease · Pathogens and Diseases
AIDS is caused by a virus whose genetic material is
HIV carries RNA, not DNA
AIDS is caused by HIV, a retrovirus with single-stranded RNA. Distractors offer double-stranded DNA or double-stranded RNA — the answer is single-stranded RNA.
Viruses cannot make their own food
A 'which statement is NOT true?' item slips in 'viruses can synthesise food by photosynthesis' — false. Viruses have no organelles and no metabolism; they depend entirely on a host.
Concept 5 of 5: Disease mechanisms and the odd facts
A few questions go past 'who causes it' to 'what does it do' — which cell a disease attacks, how a bacterium grips your teeth, what an ECG records. These are scattered single-fact recalls; collect them in one place.
Definition
The mechanism and side-facts the bank has tested:
- Dengue drops the count of platelets (thrombocytes) — causing the bleeding risk. (Not monocytes, eosinophils or neutrophils.)
- Streptococcus mutans (tooth decay) attaches to enamel by making a sticky slime layer from sugar (dietary glucose).
- An ECG (electrocardiogram) is a graphical record of the electrical activity of the heart — not the brain (EEG), kidney or cornea.
- FSSAI (Food Safety and Standards Authority of India) is the agency that enforces food-safety laws in India — not the FDA, WHO or FAO.
| Question | Answer | Note |
|---|---|---|
| Dengue reduces which blood cells? | Platelets (thrombocytes) | Causes bleeding risk; NDA 2017Q |
| Streptococcus mutans makes slime from | Sugar | Sticky glucan grips tooth enamel; NDA 2023Q |
| ECG records the activity of the | Heart | Electrical activity; brain = EEG. NDA 2019Q |
| Agency enforcing food-safety law in India | FSSAI | Not FDA, WHO or FAO. NDA 2017Q |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
NDA · Apr 2017 · Q65Easy
Example 5 · Microbiology and Disease · Pathogens and Diseases
Dengue virus causes high fever, rashes and reduces the number of a particular type of blood cells. Those blood cells are
Dengue hits platelets, not white cells
Distractors offer monocytes, eosinophils, neutrophils — all white cells. Dengue specifically lowers platelets (thrombocytes), the clotting cells, which is why bleeding is a danger.
ECG = heart, EEG = brain
An ECG (electrocardiogram) records the heart's electrical activity. The brain's electrical activity is recorded by an EEG (electroencephalogram) — a common swap.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
The disease–pathogen–type table9 rows
| Disease | Pathogen | Type |
|---|---|---|
| Malaria | Plasmodium | Protozoan |
| Sleeping sickness | Trypanosoma | ProtozoanQ Transmitted by the tsetse fly. NDA 2017. |
| Tuberculosis (TB) | Mycobacterium | BacteriumQ TB = Mycobacterium, NOT Plasmodium. The bank swaps this with malaria. NDA 2025. |
| Cholera | Vibrio cholerae | Bacterium |
| Typhoid | Salmonella typhi | BacteriumQ AIDS, dengue and COVID-19 are viral; typhoid is the bacterial one. NDA 2019, 2022. |
| Smallpox | Variola virus | VirusQ Eradicated worldwide in 1980 (WHO). NDA 2021. |
| AIDS | HIV | Virus |
| Chickenpox | Varicella zoster | Virus |
| Elephantiasis (filariasis) | Wuchereria bancrofti | Worm |
The swapped pair is the classic trap: Malaria is Plasmodium (protozoan), TB is Mycobacterium (bacterium) — never the reverse.
How diseases spread — waterborne, airborne, vector5 rows
| Route | Diseases | Key fact |
|---|---|---|
| Waterborne | Cholera, typhoid, jaundice | Spread by contaminated food or waterQ Jaundice is the waterborne answer when paired against TB (air), rabies (bite), arthritis (none). NDA 2018. |
| Cholera (route) | Vibrio cholerae | Contaminated food/water → severe watery diarrhoeaQ Not loss of memory, not a muscle disease, not genetic. NDA 2019. |
| Airborne | Tuberculosis, cold, flu, COVID-19 | Spread by respiratory droplets |
| Vector-borne | Malaria, dengue, filariasis | Carried by a mosquito or fly |
| Animal bite | Rabies | Dog or bat bite; not waterborne |
Viruses — nature and genetic material5 rows
| Statement about viruses | True or false |
|---|---|
| Need living cells to reproduce | True |
| All viruses are parasites | True |
| Can synthesize food by photosynthesis | FalseQ Viruses have no chloroplasts or organelles — they cannot photosynthesise. The 'NOT true' answer. NDA 2019. |
| Behave like chemicals outside a host | True |
| HIV genetic material | Single-stranded RNAQ HIV is a retrovirus — single-stranded RNA, not DNA. NDA 2018. |
Disease mechanisms and the odd facts4 rows
| Question | Answer | Note |
|---|---|---|
| Dengue reduces which blood cells? | Platelets (thrombocytes) | Causes bleeding risk; NDA 2017Q |
| Streptococcus mutans makes slime from | Sugar | Sticky glucan grips tooth enamel; NDA 2023Q |
| ECG records the activity of the | Heart | Electrical activity; brain = EEG. NDA 2019Q |
| Agency enforcing food-safety law in India | FSSAI | Not FDA, WHO or FAO. NDA 2017Q |
Watch out for (9)
- Malaria ↔ Plasmodium, TB ↔ Mycobacterium — never swapped→ The disease–pathogen–type table
- Typhoid is the bacterial odd-one-out→ The disease–pathogen–type table
- Sleeping sickness is a protozoan, not a worm→ The disease–pathogen–type table
- Cholera is NOT a memory, muscle or genetic disease→ How diseases spread — waterborne, airborne, vector
- TB is airborne, not waterborne→ How diseases spread — waterborne, airborne, vector
- HIV carries RNA, not DNA→ Viruses — nature and genetic material
- Viruses cannot make their own food→ Viruses — nature and genetic material
- Dengue hits platelets, not white cells→ Disease mechanisms and the odd facts
- ECG = heart, EEG = brain→ Disease mechanisms and the odd facts
Test yourself on Microbiology and Disease
10 past NDA questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.