Microbiology and Immunology

Why this topic matters for NDA

In the NDA General Ability Test (GAT), General Science carries a fixed block of marks, and Biology questions inside it are among the most predictable on the whole paper. Microbiology and immunology show up almost every year — usually as a one-line fact such as "Which disease is caused by a virus?" or "Edward Jenner is associated with which vaccine?"

These are guaranteed marks. There is no calculation, no reasoning trap — just a clear list of facts in your head. Compare this with Mathematics, where a single slip can wipe out a whole sum. Here, if you know the microbe-disease pairing, the mark is yours.

Because NDA cutoffs are tight, every such certain mark counts. Many candidates skip Biology, thinking it is "too factual to study", and then lose 3–4 easy marks to guesswork. Spend one focused hour on microbes and immunity, and you protect those marks for good.

What are microorganisms?

A microorganism (or microbe) is a living thing so small that it can only be seen under a microscope. They were first observed by Antonie van Leeuwenhoek, who built simple microscopes and called the tiny moving creatures "animalcules".

Microbes live almost everywhere — in soil, water, air, inside other living bodies, in hot springs, and even in ice. Even today, the total mass of microbes on Earth is greater than that of all plants and animals combined, which tells you just how successful these tiny organisms are. They are broadly divided into these groups:

• Bacteria — single-celled organisms without a true nucleus (prokaryotes).
• Viruses — not fully living; they reproduce only inside a host cell.
• Fungi — include moulds, yeasts and mushrooms.
• Protozoa — single-celled animal-like microbes such as Amoeba and Plasmodium.
• Algae — simple plant-like microbes such as Chlamydomonas and Spirogyra.

For the NDA exam you do not need deep biology of each group. You mainly need to know which group a named microbe belongs to and what disease or benefit it is linked with. Keep your focus on these connections rather than on long descriptions.

Bacteria: the most common microbes

Bacteria are the most abundant living things on Earth. Each bacterium is a single cell with a cell wall but no membrane-bound nucleus, which makes it a prokaryote. They are classified by shape:

• Cocci — round (e.g. Streptococcus).
• Bacilli — rod-shaped (e.g. the TB bacterium).
• Spirilla — spiral or coiled.
• Vibrio — comma-shaped (e.g. the cholera bacterium).

Useful bacteria do important jobs: Lactobacillus turns milk into curd, Rhizobium in plant roots fixes nitrogen from the air into the soil, and many bacteria in sewage break down waste. Harmful bacteria, however, cause diseases such as tuberculosis, cholera, typhoid and tetanus.

Bacteria reproduce extremely fast by simply splitting into two, a process called binary fission. Under good conditions a single bacterium can multiply into millions within hours. This is why food left in the open spoils quickly and why infections can spread so rapidly through dirty water.

Viruses: living or non-living?

A virus sits on the border between living and non-living. Outside a host it is inert like a chemical; inside a living cell it comes alive and multiplies rapidly. A virus is made of genetic material (DNA or RNA) wrapped in a protein coat called a capsid.

Because a virus has no cell machinery of its own, it must hijack a host cell to reproduce. This is why antibiotics do not cure viral diseases — there is no bacterial cell to attack.

Important viral diseases include the common cold, influenza (flu), measles, mumps, chickenpox, polio, rabies, dengue, hepatitis, AIDS and COVID-19.

Fungi, protozoa and other microbes

Fungi are simple organisms that cannot make their own food, so they live on dead matter or on other living things. Yeast is a fungus used to make bread and alcohol because it carries out fermentation, releasing carbon dioxide that makes dough rise. The mould Penicillium gave us the antibiotic penicillin. Some fungi cause diseases like ringworm and athlete's foot.

Protozoa are single-celled, animal-like microbes. The most important for NDA is Plasmodium, which causes malaria and is spread by the female Anopheles mosquito. Entamoeba causes amoebic dysentery, a serious infection of the intestines spread through contaminated food and water. Giardia is another protozoan that causes stomach trouble. Many algae, by contrast, are useful: they release oxygen during photosynthesis and form the base of food chains in ponds and oceans.

Common diseases and their causes

This is the single most tested area. Learn which microbe causes which disease, and how it spreads.

Bacterial diseases

• Tuberculosis (TB) — bacterium; affects the lungs.
• Cholera — bacterium; spread by contaminated water and food.
• Typhoid — bacterium; spread through contaminated food/water.
• Tetanus — bacterium; enters through wounds.

Viral diseases

• Polio, measles, mumps, chickenpox — spread by contact or droplets.
• Dengue — virus spread by the Aedes mosquito.
• AIDS — caused by the HIV virus.
• Rabies — spread by the bite of an infected animal (e.g. a dog).

Protozoan diseases

• Malaria — Plasmodium, spread by the female Anopheles mosquito.

Immunity: the body's defence force

Immunity is the body's ability to resist and fight off disease-causing microbes. It works like a built-in army. There are two broad types:

• Innate (natural) immunity — present from birth. The skin, stomach acid, mucus, tears and white blood cells form the first line of defence against any invader.
• Acquired (adaptive) immunity — developed during life after the body meets a particular microbe. It is specific and has memory.

When a harmful microbe enters, the body recognises foreign substances on it called antigens. In response, special white blood cells (lymphocytes) produce antibodies that attack and neutralise the microbe.

Vaccines and how they work

A vaccine contains weakened, dead, or part of a microbe. When injected, it does not cause real disease but tricks the body into making antibodies and "memory cells". If the real microbe ever attacks later, the body recognises it instantly and destroys it. This is the principle of vaccination or immunisation.

Edward Jenner developed the first vaccine in 1796, against smallpox, using material from cowpox. Thanks to worldwide vaccination, smallpox has been completely eradicated. Louis Pasteur later developed vaccines for rabies and anthrax and gave us the process of pasteurisation to kill germs in milk.

Active vs passive immunity

• Active immunity — the body makes its own antibodies (e.g. after a vaccine or after recovering from a disease). It is long-lasting.
• Passive immunity — ready-made antibodies are given from outside (e.g. a baby getting antibodies through mother's milk, or an injection of antiserum). It acts fast but is short-lived.

Useful microbes in daily life

It is easy to think of microbes only as enemies, but most are friends that we depend on every day.

• Food: Lactobacillus turns milk into curd; yeast helps make bread, idli and dosa batter, and alcohol.
• Medicine: Penicillium fungus gives the antibiotic penicillin; many antibiotics come from microbes.
• Agriculture: Rhizobium bacteria in the root nodules of leguminous plants fix atmospheric nitrogen, naturally enriching the soil.
• Environment: Bacteria and fungi act as decomposers, breaking down dead plants and animals and recycling nutrients. They also clean sewage in treatment plants and help produce biogas.
• Industry: microbes are used to produce vinegar, cheese, vitamins and many medicines on a large scale.

Worked example

Let's apply the microbe-disease logic to a typical question.

Notice how knowing just two facts — the cause and the carrier — lets you answer several sub-questions confidently.

Common mistakes to avoid

A few errors cost candidates easy marks every year. Watch out for these.

Previous-year style practice

Try this NDA-style question before you check the answer.