CDS Biology · Microbiology and Disease
Pathogens and Their Spread
Which diseases are caused by bacteria, viruses, fungi, protozoa and worms, what makes viruses different, how diseases spread from one person to another, and what endemic, epidemic and pandemic mean.
Why this matters
Twelve CDS questions from 2016 (II) to 2026 (II). Sleeping sickness (Trypanosoma), tuberculosis and dengue have each come up twice. Most ask which germ causes a disease or how a disease spreads.
Concept 1 of 4: Diseases by kind of pathogen
A germ that causes disease is a pathogen. Pathogens come in five kinds: bacteria, viruses, fungi, protozoa (single-celled animals) and worms. Knowing which kind causes a disease tells you how to treat it: antibiotics work on bacteria but not on viruses. So learn a few diseases under each kind.
Definition
Disease-causing organisms (NCERT Class 9, Why Do We Fall Ill?, and Class 12, Human Health and Disease):
- Bacteria: tuberculosis, cholera, typhoid, whooping cough, anthrax, and peptic ulcers, which are caused by Helicobacter pylori (found by Marshall and Warren).
- Viruses: common cold, influenza, dengue, polio, measles, chickenpox, AIDS.
- Fungi: many skin infections, such as ringworm and athlete's foot.
- Protozoa: malaria (Plasmodium), kala-azar (Leishmania), sleeping sickness (Trypanosoma, spread by the tsetse fly), amoebic dysentery (Entamoeba).
- Worms: elephantiasis or filariasis (Wuchereria), ascariasis (Ascaris).
| Disease | Pathogen | Kind |
|---|---|---|
| Tuberculosis | Mycobacterium tuberculosis | Bacterium |
| Peptic ulcer | Helicobacter pylori | BacteriumQ CDS 2020 (II): besides excess acid, peptic ulcers are caused by Helicobacter pylori. |
| Typhoid | Salmonella typhi | Bacterium |
| Dengue | Dengue virus | Virus |
| Athlete's foot, ringworm | Fungi | Fungus |
| Malaria | Plasmodium | Protozoan |
| Sleeping sickness | Trypanosoma | ProtozoanQ Asked twice: CDS 2019 (II) and 2021 (I). Leishmania causes kala-azar, not sleeping sickness. |
| Kala-azar | Leishmania | Protozoan |
| Elephantiasis | Wuchereria | Worm |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q10Easy
Example 1 · Microbiology and Disease · Pathogens and Their Spread
Which one among the following pairs of diseases and their types is not correctly matched ?
Trypanosoma and Leishmania
Both are protozoa spread by flies, which is why they are paired in options. Trypanosoma causes sleeping sickness (tsetse fly); Leishmania causes kala-azar (sandfly).
Ulcers are not only from acid
A peptic ulcer is not caused by acid alone. The bacterium Helicobacter pylori is a major cause, so antibiotics are part of the cure. E. coli and Salmonella are other bacteria.
Concept 2 of 4: Viruses
A virus is not a cell. It is a little genetic material, DNA or RNA, packed in a protein coat, and it can multiply only inside a living cell, using that cell's machinery. RNA is less stable than DNA and is copied with more mistakes, so RNA viruses change fast, which is why the flu vaccine must be updated so often.
Definition
Facts about viruses:
- A virus multiplies only inside a living host cell; outside it, it shows no life activity.
- Its genetic material is either DNA or RNA, never both. RNA viruses include HIV, influenza, polio, measles and dengue; DNA viruses include hepatitis B, adenovirus and vaccinia (the pox virus).
- RNA is less stable than DNA and is copied without proofreading, so RNA viruses mutate faster and evolve quickly.
- HIV causes AIDS. It destroys the helper T lymphocytes (a kind of white blood cell), so the body's immunity falls and ordinary infections become dangerous.
- Dengue virus is a flavivirus, an RNA virus spread by the Aedes mosquito.
| Virus | Genetic material | Fact to remember |
|---|---|---|
| HIV | RNA | Destroys helper T lymphocytes; causes AIDS |
| Dengue virus | RNA | A flavivirus spread by the Aedes mosquitoQ CDS 2024 (I): dengue virus is a flavivirus, an RNA virus; the right option was printed as 'Flavi-ribo virus'. |
| Influenza virus | RNA | Changes fast, so the vaccine is updated |
| Polio virus | RNA | Prevented by oral and injected vaccines |
| Hepatitis B virus | DNA | Spread through blood |
| Adenovirus | DNA | Causes colds and eye infections |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2016 · CDS (II) 2016 — General Knowledge · Q4Easy
Example 2 · Microbiology and Disease · Pathogens and Their Spread
The HIV virus weakens the immunity of a person because it destroys
HIV attacks lymphocytes, not red cells or platelets
HIV destroys helper T lymphocytes, the white blood cells that organise the immune response. Red blood cells (erythrocytes), platelets and mast cells are not its target.
Why RNA viruses change fast
The reason is chemical: RNA is less stable than DNA and its copies are not checked. 'Viruses live only inside cells' is true of every virus, so it cannot explain why RNA viruses mutate faster.
Concept 3 of 4: How diseases spread
A germ must travel from a sick person to a healthy one. Some ride on the tiny droplets from a cough or sneeze, so they spread fastest where people crowd together. Some travel in dirty water or food. Some are carried by an animal, usually an insect, that bites one person and then another; that animal is called a vector.
Definition
Routes of spread (NCERT Class 9):
- Air (droplets) from coughs and sneezes: tuberculosis, common cold, influenza, pneumonia. Crowded, poorly aired places raise the risk.
- Water and food: cholera, typhoid, hepatitis A.
- Vectors (animals that carry the germ): the female Anopheles mosquito (malaria), Aedes (dengue, chikungunya), Culex (filariasis), the tsetse fly (sleeping sickness), the sandfly (kala-azar).
- Sexual contact and blood: AIDS, syphilis, hepatitis B.
- Mother to baby: HIV, and malaria, since Plasmodium can cross the placenta (congenital malaria).
- Only female mosquitoes bite: they need a blood meal to mature their eggs. Males feed on plant juices.
| Route | Examples |
|---|---|
| Air (droplets from coughs and sneezes) | Tuberculosis, common cold, influenzaQ CDS 2018 (I): crowded places raise the risk of tuberculosis because its bacteria spread in droplets; Statement II explains Statement I. |
| Water and food | Cholera, typhoid, hepatitis A |
| Female Anopheles mosquito | MalariaQ CDS 2026 (II): only female Anopheles take human blood, and Plasmodium can cross the placenta; both statements are correct. |
| Aedes mosquito | Dengue, chikungunya |
| Culex mosquito | Filariasis (elephantiasis) |
| Tsetse fly | Sleeping sickness |
| Sexual contact and blood | AIDS, hepatitis B, syphilis |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q42Easy
Example 3 · Microbiology and Disease · Pathogens and Their Spread
Which one of the following is an example of airborne disease ?
Three mosquitoes, three diseases
Anopheles carries malaria, Aedes carries dengue, and Culex carries filariasis. Malaria and dengue are mosquito-borne, not airborne; cholera is waterborne.
Visiting crowds is a risk, not a cause
Crowded places do not cause tuberculosis by themselves. They raise the risk because the TB bacteria travel in droplets from coughs and sneezes, so the droplet route explains why crowds matter.
Concept 4 of 4: Endemic, epidemic and pandemic
These words describe how many people catch a disease, and where. If a disease is always present in a place at about the same level, it is endemic there. If cases suddenly shoot up far above that level, it is an epidemic. If that rise spreads across countries and continents, it is a pandemic.
Definition
Words for how a disease occurs:
- Endemic: always present in a place at a steady, expected level, kept up without input from outside; for example malaria and filariasis in parts of India.
- Epidemic: a sudden rise in cases, well above the usual level, in a region; cholera, malaria and smallpox have all caused epidemics.
- Pandemic: an epidemic that spreads across many countries and continents; for example the 1918 influenza and COVID-19 (declared in 2020).
- Outbreak: an epidemic in a small, limited area.
- Sporadic: scattered cases now and then, with no pattern.
- Elephantiasis builds up slowly over years in the places where it is found; it does not break out suddenly, so it is a chronic, endemic disease, not an epidemic one.
| Term | Meaning | Example |
|---|---|---|
| Endemic | Steady level in one area | Malaria in parts of India |
| Epidemic | Sudden rise above the usual level | A cholera outbreak after a flood |
| Pandemic | Epidemic across continents | COVID-19 |
| Sporadic | Scattered, occasional cases | A few tetanus cases |
| Chronic, endemic disease | Slow, long-lasting, no sudden outbreaks | ElephantiasisQ CDS 2020 (I): of cholera, malaria, smallpox and elephantiasis, elephantiasis is not an epidemic disease. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q97Easy
Example 4 · Microbiology and Disease · Pathogens and Their Spread
An infection constantly maintained at a baseline level in a geographic area without external input is known as
Endemic is steady, epidemic is a spike
Endemic means a disease is always there at about the same level. Epidemic means a sudden spike. A disease can be endemic in a place and still have epidemics there.
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)
Diseases by kind of pathogen9 rows
| Disease | Pathogen | Kind |
|---|---|---|
| Tuberculosis | Mycobacterium tuberculosis | Bacterium |
| Peptic ulcer | Helicobacter pylori | BacteriumQ CDS 2020 (II): besides excess acid, peptic ulcers are caused by Helicobacter pylori. |
| Typhoid | Salmonella typhi | Bacterium |
| Dengue | Dengue virus | Virus |
| Athlete's foot, ringworm | Fungi | Fungus |
| Malaria | Plasmodium | Protozoan |
| Sleeping sickness | Trypanosoma | ProtozoanQ Asked twice: CDS 2019 (II) and 2021 (I). Leishmania causes kala-azar, not sleeping sickness. |
| Kala-azar | Leishmania | Protozoan |
| Elephantiasis | Wuchereria | Worm |
Viruses6 rows
| Virus | Genetic material | Fact to remember |
|---|---|---|
| HIV | RNA | Destroys helper T lymphocytes; causes AIDS |
| Dengue virus | RNA | A flavivirus spread by the Aedes mosquitoQ CDS 2024 (I): dengue virus is a flavivirus, an RNA virus; the right option was printed as 'Flavi-ribo virus'. |
| Influenza virus | RNA | Changes fast, so the vaccine is updated |
| Polio virus | RNA | Prevented by oral and injected vaccines |
| Hepatitis B virus | DNA | Spread through blood |
| Adenovirus | DNA | Causes colds and eye infections |
How diseases spread7 rows
| Route | Examples |
|---|---|
| Air (droplets from coughs and sneezes) | Tuberculosis, common cold, influenzaQ CDS 2018 (I): crowded places raise the risk of tuberculosis because its bacteria spread in droplets; Statement II explains Statement I. |
| Water and food | Cholera, typhoid, hepatitis A |
| Female Anopheles mosquito | MalariaQ CDS 2026 (II): only female Anopheles take human blood, and Plasmodium can cross the placenta; both statements are correct. |
| Aedes mosquito | Dengue, chikungunya |
| Culex mosquito | Filariasis (elephantiasis) |
| Tsetse fly | Sleeping sickness |
| Sexual contact and blood | AIDS, hepatitis B, syphilis |
Endemic, epidemic and pandemic5 rows
| Term | Meaning | Example |
|---|---|---|
| Endemic | Steady level in one area | Malaria in parts of India |
| Epidemic | Sudden rise above the usual level | A cholera outbreak after a flood |
| Pandemic | Epidemic across continents | COVID-19 |
| Sporadic | Scattered, occasional cases | A few tetanus cases |
| Chronic, endemic disease | Slow, long-lasting, no sudden outbreaks | ElephantiasisQ CDS 2020 (I): of cholera, malaria, smallpox and elephantiasis, elephantiasis is not an epidemic disease. |
Watch out for (7)
- Trypanosoma and Leishmania→ Diseases by kind of pathogen
- Ulcers are not only from acid→ Diseases by kind of pathogen
- HIV attacks lymphocytes, not red cells or platelets→ Viruses
- Why RNA viruses change fast→ Viruses
- Three mosquitoes, three diseases→ How diseases spread
- Visiting crowds is a risk, not a cause→ How diseases spread
- Endemic is steady, epidemic is a spike→ Endemic, epidemic and pandemic
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