Germs and infectious disease
A pathogen is a germ that causes disease. There are four main kinds: bacteria (single cells, e.g. tuberculosis, typhoid), viruses (tiny, live inside our cells, e.g. flu, measles, COVID-19), fungi (e.g. ringworm) and parasites (e.g. the malaria parasite, worms).
Germs spread by air (coughs), water and food, touch, body fluids and animal bites (mosquitoes). An infectious disease can spread; a non-infectious one (diabetes) cannot.
Line 1: barriers (outer defence)
- Skin: a tough, dry wall. A cut opens a door.
- Mucus and tiny hairs in the nose and airways trap germs; we cough or sneeze them out.
- Tears and saliva contain an enzyme that breaks germs.
- Stomach acid kills most germs in food.
- Blood clotting quickly seals a wound.
Line 2: innate (non-specific) immunity
This line is present from birth and attacks any germ in the same way. It is fast (minutes to hours).
- Inflammation: the injured area becomes red, warm, swollen and painful because more blood flows there, bringing white blood cells.
- Phagocytes (a type of white blood cell) swallow and digest germs. This is phagocytosis.
- Fever: a higher body temperature slows many germs.
Line 3: adaptive (specific) immunity
Lymphocytes are white blood cells made in bone marrow, trained in the thymus and lymph nodes. They recognise the germ's antigen (a marker molecule on its surface).
- B cells make antibodies: Y-shaped proteins whose tips fit one antigen only. Antibodies clump germs, block viruses and mark them for phagocytes. This is humoral (fluid) immunity.
- T cells: helper T cells direct the attack; killer T cells destroy our own cells that are infected by viruses. This is cell-mediated immunity.
The lymphatic system (lymph, lymph nodes, spleen) carries and houses these cells. Swollen "glands" in the neck during a cold are busy lymph nodes.
Memory, vaccines and types of immunity
After an infection some lymphocytes become memory cells. A second attack by the same germ is met quickly and strongly.
A vaccine contains a dead or weakened germ, or just a harmless part of it (or instructions to make that part, as in mRNA vaccines). It makes memory cells without the disease. When most people are vaccinated, germs cannot spread easily and even unvaccinated babies are protected: herd immunity. India's Universal Immunisation Programme and worldwide polio drives are examples.
| Active | Passive | |
|---|---|---|
| What | Your body makes antibodies | Ready-made antibodies are given |
| Example | Infection, vaccine | Mother's milk, anti-snake-venom injection |
| Lasts | Long (memory) | Short (weeks) |
Antibiotics kill bacteria but do NOT work on viruses. Overusing them breeds resistant bacteria.
When immunity goes wrong
- Allergy: the body over-reacts to something harmless (pollen, dust, peanuts). Severe allergy (anaphylaxis) is an emergency.
- Autoimmune disease: the immune system attacks the body's own cells (type 1 diabetes, rheumatoid arthritis).
- Immunodeficiency: the system is weak. HIV destroys helper T cells, leading to AIDS, so ordinary germs become dangerous.
Good food, sleep, exercise, hygiene and vaccines keep the immune system strong.
Key formulas and definitions
- Pathogen = disease-causing germ (bacteria, virus, fungus, parasite)
- Antigen = marker on the germ; antibody = Y-shaped protein that fits it
- Innate = fast, general; adaptive = slow first time, specific, has memory
- Vaccine → memory cells → fast second response
- Active immunity (own antibodies) vs passive immunity (given antibodies)
Worked examples
1. A child cuts her knee. By evening it is red, warm and swollen. Explain.
This is inflammation (line 2). Blood vessels widen, more blood reaches the cut (red, warm), fluid leaks out (swelling), and phagocytes arrive to eat germs that entered through the broken skin (line 1 failed at the cut).
2. Why does a person rarely get measles twice?
The first infection leaves memory B and T cells for the measles antigen. On a second attack they multiply fast and make antibodies within a day or two, so the virus is removed before illness develops.
3. A doctor refuses to give antibiotics for a common cold. Why is this right?
A cold is caused by a virus. Antibiotics act only on bacteria, so they would not help. Using them needlessly helps bacteria become resistant.
4. A snake-bite victim gets anti-venom, not a vaccine. Why?
The venom acts within hours. A vaccine needs days to weeks to build memory. Anti-venom gives ready-made antibodies at once: passive immunity.
Common mistakes
- Thinking antibiotics cure viral diseases like flu or colds. They work only on bacteria.
- Mixing up antigen and antibody. Antigen is on the germ; antibody is made by our B cells.
- Thinking phagocytes are specific. They eat any germ; only B and T cells are specific.
- Thinking vaccines cure an illness you already have. They prevent it by building memory in advance.