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
Intuition
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.
- Tuberculosis is caused by Mycobacterium — a bacterium.
- Malaria is caused by Plasmodium — a protozoan (single-celled, with a nucleus).
- Smallpox is caused by the Variola virus — a virus.
- Elephantiasis is caused by Wuchereria bancrofti — a worm (filarial roundworm).
Answer:(a) bacterium, (b) protozoan, (c) virus, (d) worm.
Practice this concept4 quick reps
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.What kind of pathogen causes malaria?
- 2.What kind of pathogen causes AIDS?
- 3.What kind of pathogen causes elephantiasis?
- 4.Is a bacterium a prokaryote or a eukaryote?
Concept 2 of 5
The disease–pathogen–type table
Intuition
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
Try it yourself
An exam lists the pair 'Typhoid : Trypanosoma'. Is it correctly matched? If not, give the right pathogen for each name.
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which organism causes sleeping sickness?
- 2.Which organism causes tuberculosis?
- 3.Which bacterium causes typhoid?
- 4.Which virus caused smallpox?
- 5.Of AIDS, dengue, COVID-19 and typhoid — which is bacterial?
From the bank · past-year question
Which of the following pairs is/are correctly matched?
1. Malaria : Mycobacterium
2. TB : Plasmodium
Select the answer using the code given below:
[Q114 · Apr · 2025]
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
Intuition
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
Try it yourself
From this list — tuberculosis, jaundice, rabies, arthritis — which one is waterborne, and why are the others not?
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Cholera is caused by consuming ___.
- 2.Name a waterborne disease.
- 3.How does tuberculosis spread?
- 4.How does rabies spread?
From the bank · past-year question
Which one of the following statements regarding Cholera is correct?
[Q150 · Apr · 2019]
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
Intuition
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
Try it yourself
Which statement is NOT true of viruses: (a) they need a host to reproduce, (b) they are parasites, (c) they make food by photosynthesis, (d) they act like chemicals outside a host?
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.What is the genetic material of HIV?
- 2.Can a virus reproduce outside a living cell?
- 3.Can a virus photosynthesise?
- 4.A virus is a protein coat around what?
From the bank · past-year question
AIDS is caused by a virus whose genetic material is
[Q69 · Apr · 2018]
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
Intuition
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
Try it yourself
Dengue is dangerous partly because it sharply lowers the count of one type of blood cell. Which one, and what does that cause?
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Dengue reduces the count of which blood cells?
- 2.Streptococcus mutans builds its slime layer from what?
- 3.An ECG records the electrical activity of which organ?
- 4.Which agency enforces food-safety laws in India?
From the bank · past-year question
Dengue virus causes high fever, rashes and reduces the number of a particular type of blood cells. Those blood cells are
[Q65 · Apr · 2017]
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
Mastery check — 5 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
Which one of the following is a bacterium that causes disease in the human body?
[Q93 · Sep · 2019]
Which one of the following is a waterborne disease?
[Q61 · Sep · 2018]
Which one of the following statements regarding viruses is true?
[Q94 · Sep · 2019]
Which one of the following statements regarding Electrocardiogram is correct?
[Q147 · Apr · 2019]
Which one of the following is caused by a bacterial pathogen?
[Q77 · Apr · 2022]
Drill every past-year question on this subtopic
13 questions from the bank — paginated, with cart and Word-export support.