Living things too small to see without a microscope are microorganisms or microbes. Most are harmless and many are useful — they make curd, bread and medicines and keep the soil fertile. A few are pathogens that cause disease. Exams ask which microbe causes a disease, how it spreads, which medicine or vaccine works, and who discovered it.
On this page you will learn:
- The five groups of microorganisms
- Useful microbes, food spoilage and shelf life
- Diseases, their germs and how they spread
- From 'bad air' to germ theory: the Black Death and the great scientists
- Vaccines and antibiotics
5
Groups of microbes
Bacteria, fungi, protozoa, algae, viruses
1796
First vaccine
Edward Jenner, smallpox
2014
India certified polio-free
By the WHO
Kinds of microorganisms
Rule in one line: viruses are the odd ones out — they multiply only inside a living host cell.
| Group | Features | Examples |
|---|---|---|
| Bacteria | Single cells without a true nucleus | Lactobacillus, Rhizobium, TB and cholera germs |
| Fungi | Single-celled or thread-like; no chlorophyll | Yeast, bread mould, Penicillium, ringworm fungus |
| Protozoa | Single-celled, animal-like | Amoeba, Paramecium, Plasmodium |
| Algae | Plant-like; make food by photosynthesis | Chlamydomonas, Spirogyra |
| Viruses | Not cells; multiply only inside living cells | Cold, flu, polio, HIV, measles, chickenpox viruses |
CAUSED BY BACTERIA
- Tuberculosis (TB)
- Typhoid, cholera
- Tetanus, plague
- Pneumonia (usually)
CAUSED BY VIRUSES
- Common cold, flu, Covid-19
- Chickenpox, measles
- Polio, dengue, rabies
- AIDS (HIV), hepatitis
QUICK CHECK
PYQ · Punjab Police Constable 2021 — Which of the following disease is caused by fungi?
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Answer: A — Ring Worm
Ringworm is a fungal skin infection, not a worm. Cholera and TB are bacterial; Covid-19 is viral.
QUICK CHECK
PYQ · Punjab Police SI 2016 — Which of the following is not a viral disease?
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Answer: C — Pneumonia
Pneumonia is usually bacterial, though it can also be viral. Chickenpox, AIDS and the common cold are always viral, so pneumonia is the answer.
Microbes in food: friends and spoilers
Rule in one line: microbes need water and warmth — take these away and food keeps longer.
| Use | Microbe | What happens |
|---|---|---|
| Curd | Lactobacillus (bacterium) | Turns milk into curd |
| Bread, idli, bhatura | Yeast (Saccharomyces, a fungus) | CO2 gas makes dough rise |
| Alcohol | Yeast | Sugar becomes alcohol by fermentation |
| Antibiotics | Fungi and bacteria | Penicillin from Penicillium; streptomycin from Streptomyces |
| Soil fertility | Rhizobium; blue-green algae | Fix air nitrogen in the soil |
| Cleaning the environment | Decomposer bacteria and fungi | Break dead matter into manure |
1
Drying
Removes water. Grains, pulses and biscuits have low water activity, so they keep for months.
2
Salt and sugar
Pull water out of microbes. Used in pickles, jams and squashes.
3
Cold
A fridge slows the growth of microbes; it does not kill them.
4
Pasteurisation
Milk is heated to about 70 °C for 15–30 seconds, then chilled quickly.
Ways to keep food safe. Fresh, moist foods such as milk, meat and fruit are perishable.
QUICK CHECK
PYQ · Punjab Police Constable 2025 — Which of the following types of organisms is responsible for causing food infection upon ingestion?
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Answer: C — Microorganisms
Food infection means swallowing live microorganisms that then grow in the body. Hormones, enzymes and toxins are not organisms; a toxin causes food intoxication.
QUICK CHECK
PYQ · Punjab Police Constable 2025 — Which of the following is a likely cause of food poisoning in humans?
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Answer: C — Salmonella
Salmonella causes food poisoning. Lactobacillus makes curd, Saccharomyces is baker's yeast, and Pseudomonas denitrificans is a soil bacterium that turns nitrates into nitrogen gas.
QUICK CHECK
PYQ · Punjab Police Constable 2025 — Which of the following factors prolongs the shelf life of non-perishable foods?
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Answer: A — They have low water activity, which inhibits microbiological growth.
Microbes need free water to grow. Dry foods such as grains and sugar have little of it, so they last long without a fridge. Moisture helps microbes; it does not stop them.
Diseases and how they spread
Rule in one line: learn each disease with its germ and its carrier.
| Disease | Caused by | Spreads through |
|---|---|---|
| Tuberculosis (TB) | Bacterium | Air |
| Typhoid | Bacterium (Salmonella Typhi) | Contaminated water and food |
| Cholera | Bacterium | Contaminated water and food |
| Tetanus | Bacterium | Wounds dirtied by soil or rust |
| Common cold, flu | Virus | Air |
| Measles, chickenpox | Virus | Air, contact |
| Polio | Virus | Contaminated food and water |
| Hepatitis A | Virus | Contaminated water |
| AIDS | Virus (HIV) | Sexual contact, infected blood, mother to child |
| Rabies | Virus | Bite of an infected dog or other animal |
| Dengue | Virus | Aedes mosquito |
| Malaria | Protozoan (Plasmodium) | Female Anopheles mosquito |
| Ringworm | Fungus | Contact, shared towels and combs |
Malaria and dengue: germ and carrier
Plasmodium, the malaria protozoan, as small purple rings inside red blood cells
Michael Zahniser, Wikimedia Commons · Public domain
Aedes aegypti, the dengue mosquito: note the white bands on its legs. It bites by day.
James Gathany (CDC), Wikimedia Commons · Public domain
QUICK CHECK
PYQ · PPSC OHE 2016 — Malaria is caused by:
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Answer: C — Protozoa
Malaria is caused by the protozoan Plasmodium. The female Anopheles mosquito carries it from person to person.
QUICK CHECK
PYQ · PPSC OHE 2016 — 'Dengue' is caused by:
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Answer: B — Aedes aegypti mosquito
Strictly, the cause is the dengue virus; Aedes aegypti is the mosquito that carries it. 'Virus' is not an option, so Aedes aegypti is the best answer. Anopheles carries malaria.
KEY TERM
HIV and AIDS
HIV (Human Immunodeficiency Virus) attacks white blood cells called lymphocytes (helper T-cells). As they are destroyed, the body's defence becomes weak. Then common infections become deadly. This last stage is AIDS — Acquired Immuno Deficiency Syndrome.
HIV enters blood→Lymphocytes destroyed→Immunity falls→AIDS
QUICK CHECK
PYQ · Punjab Police Constable 2024 — The full name of AIDS is ________.
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Answer: C — Acquired Immuno Deficiency Syndrome
The 'D' stands for Deficiency — a lack of immunity — not 'Disease'.
QUICK CHECK
PYQ · PPSC OHE 2016 — The HIV virus weakens the immunity of a person because it destroys
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Answer: D — Lymphocytes
HIV kills helper T-lymphocytes, the cells that organise the body's defence. Erythrocytes carry oxygen; platelets help clotting.
Diseases of animals and plants; tests to knowTap to open
| Disease | Host | Caused by |
|---|---|---|
| Anthrax | Cattle and humans | Bacterium |
| Foot-and-mouth disease | Cattle | Virus |
| Citrus canker | Citrus plants | Bacterium |
| Rust of wheat | Wheat | Fungus |
| Yellow vein mosaic | Okra (bhindi) | Virus |
- The Widal test is a blood test used to confirm typhoid.
- A blood smear seen under the microscope shows the malaria parasite in red blood cells.
- Mosquitoes also carry filaria (Culex), Japanese encephalitis and yellow fever.
From 'bad air' to germs: a short history
Rule in one line: people once blamed bad air; Pasteur and Koch proved that germs cause disease.
THE BLACK DEATH
In 1347–1351 a plague pandemic swept Europe and killed a very large share of its people — often given as a third to a half or more. Plague is caused by a bacterium (Yersinia pestis) carried by fleas on rats.
MILESTONES AGAINST DISEASE
- 1796Edward Jenner
Used cowpox to protect against smallpox — the first vaccine.
- 1857Louis Pasteur
Showed that fermentation is done by microbes; later gave pasteurisation and the rabies vaccine (1885).
- 1882Robert Koch
Found the TB bacterium. World TB Day, 24 March, marks this.
- 1897Ronald Ross
In India, showed that the Anopheles mosquito carries the malaria parasite; Nobel Prize 1902.
- 1928Alexander Fleming
Discovered penicillin from the mould Penicillium.
Three names exams ask
Edward Jenner. Note the cows and milkmaid: he saw that milkmaids who caught cowpox did not get smallpox. 'Vaccine' comes from vacca, Latin for cow.
John Raphael Smith, Wikimedia Commons · Public domain
Louis Pasteur, the father of germ theory: fermentation, pasteurisation of milk and the rabies vaccine.
Paul Nadar, Wikimedia Commons · Public domain
Ronald Ross, born in Almora, India. He traced the malaria parasite inside the Anopheles mosquito (1897).
Unknown author, Wikimedia Commons · Public domain
QUICK CHECK
PYQ · Punjab Police SI 2016 — The Black Death was one of the most devastating pandemics in human history that killed around 60% of Europe's population due to which of the following?
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Answer: B — Plague
The Black Death was plague, a bacterial disease spread by rat fleas. The death share is uncertain; some historians give figures as high as the 60% in the question.
QUICK CHECK
PYQ · Punjab Police SI 2021 — The theory that held that diseases such as cholera, chlamydia, plague etc. were caused by pollution or a noxious form of "bad air", also known as "night air", and that diseases were the product of environmental factors is:
Tap an option to answer
Answer: B — The Miasma Theory
'Bad air' is the miasma theory. Germ theory, which replaced it, says living microbes cause disease.
Vaccines and antibiotics
Rule in one line: a vaccine prevents disease in advance; an antibiotic treats a bacterial infection after it starts.
KEY TERM
Vaccine
A vaccine contains dead or weakened germs, or just a harmless part of them such as a protein. It makes the body produce antibodies and remember the germ. A real infection later is fought off quickly. So vaccination is preventive medicine. Smallpox has been wiped out worldwide, and India was certified polio-free in 2014.
Vaccine given→Antibodies made→Memory cells stay→Germ beaten fast
| Vaccine | Protects against |
|---|---|
| BCG | Tuberculosis |
| OPV (drops) / IPV (injection) | Polio |
| DPT | Diphtheria, pertussis (whooping cough), tetanus |
| MMR | Measles, mumps, rubella |
| Hepatitis A, Hepatitis B | Liver infections |
| Varicella | Chickenpox |
QUICK CHECK
PYQ · Punjab Police SI 2023 — Vaccination is an example of ________.
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Answer: C — preventive medicine
A vaccine is given to healthy people before they fall ill, so it prevents disease. Curative medicine treats a disease that has already started.
QUICK CHECK
PYQ · Punjab Police Constable 2021 — Which disease among the following does not have a vaccine?
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Answer: D — HIV
Chickenpox, hepatitis A and hepatitis B all have vaccines. HIV keeps changing its surface, and no vaccine has been approved yet.
ANTIBIOTICS DO NOT WORK ON VIRUSES
An antibiotic is a medicine, first made by microbes, that kills or stops bacteria. It blocks processes that only bacteria have. Viruses live inside our own cells, so antibiotics are useless against the cold, flu or chickenpox. Taking them when not needed helps bacteria become resistant.
| Antibiotic | Found | Known for |
|---|---|---|
| Penicillin | 1928, Alexander Fleming, from Penicillium mould | The first antibiotic |
| Streptomycin | 1943, from the soil bacterium Streptomyces | First drug that cured TB |
| Chloramphenicol | 1947, from a Streptomyces bacterium | Used against typhoid |
QUICK CHECK
PYQ · Punjab Police Constable 2024 — Which of the following diseases is successfully treated with Streptomycin?
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Answer: D — Tuberculosis
Streptomycin (1943) was the first antibiotic that cured TB. Yellow fever is viral and malaria is protozoan, so an antibiotic cannot cure them.
QUICK CHECK
PYQ · Punjab Police SI 2023 — When was Chloramphenicol discovered?
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Answer: D — 1947
Chloramphenicol was isolated from a Streptomyces soil bacterium in 1947. It was soon used to treat typhoid.
QUICK CHECK
PYQ · Punjab Police SI 2023 — Which of these is a bactericidal type of antibiotic?
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Answer: C — Aminoglycosides
Aminoglycosides such as streptomycin kill bacteria. Tetracycline, erythromycin and chloramphenicol only stop their growth (bacteriostatic).
How to prevent infectious diseases; science in the newsTap to open
- Drink boiled or purified water; eat freshly cooked food.
- Wash hands with soap before eating and after using the toilet.
- Do not let water collect in coolers, tyres and pots — it breeds mosquitoes.
- Cover the mouth and nose when coughing or sneezing.
- Get every vaccine on time; use mosquito nets and repellents.
- In April 2024 the President of India launched India's first home-grown gene therapy for cancer, a CAR-T cell therapy, at IIT Bombay. It was developed with Tata Memorial Hospital.
Traps and practice
Last revision — where the options try to fool you.
COMMON TRAPS
- Ringworm is a fungus, not a worm.
- Malaria is caused by a protozoan; the female Anopheles mosquito only carries it.
- Dengue is a virus carried by Aedes, which bites by day. Anopheles carries malaria.
- AIDS: the 'D' is Deficiency, not Disease. HIV destroys lymphocytes, not red cells.
- Pneumonia is the usual 'not viral' answer: it is mostly bacterial.
- Food lasts longer with low water activity, not high moisture.
- 'Bad air' = miasma theory; germs = germ theory (Pasteur, Koch).
- Antibiotics kill bacteria, not viruses. Streptomycin → TB; chloramphenicol → 1947.
CHECK YOURSELF
Why does a doctor not give antibiotics for a common cold?
Show answer
The common cold is caused by a virus. Antibiotics act only on bacteria, so they will not help. Taking them anyway can make bacteria resistant.
FLASHCARDS
Microbe and disease flashcards
- Curd
- Lactobacillus
- Food poisoning germ
- Salmonella
- Malaria parasite
- Plasmodium (protozoan)
- Dengue carrier
- Aedes aegypti
- Cells HIV destroys
- Lymphocytes
- Black Death
- Plague
- First vaccine
- Jenner, smallpox, 1796
- First drug to cure TB
- Streptomycin
- Test for typhoid
- Widal test
QUICK BYTES
- Five microbe groups: bacteria, fungi, protozoa, algae, viruses
- TB, typhoid, cholera, plague: bacteria; ringworm: fungus
- Cold, chickenpox, polio, AIDS, dengue: viruses
- Malaria: Plasmodium via female Anopheles
- Low water activity = long shelf life; Salmonella = food poisoning
- Vaccines prevent; no approved HIV vaccine yet
- Penicillin 1928, streptomycin 1943 (TB), chloramphenicol 1947