Pathogens, Immunity and Vaccination
Pathogens and disease
A pathogen is a microorganism that causes communicable (infectious) disease. There are four types: bacteria, viruses, fungi and protists. This topic covers how they spread, how the body defends itself, and how we prevent and treat disease.
The four types of pathogen
| Type | How it causes harm | Example disease |
|---|---|---|
| Bacteria | Reproduce rapidly and release toxins | Salmonella, gonorrhoea |
| Virus | Invade cells and reproduce inside, bursting the cell | Measles, HIV, COVID-19 |
| Fungi | Grow on/in tissue | Rose black spot, athlete's foot |
| Protist | Often spread by a vector | Malaria (spread by mosquitoes) |
How pathogens spread
- Direct contact (e.g. touching, some STIs)
- Water (e.g. contaminated water)
- Air / droplets (e.g. coughs and sneezes)
- Vectors (animals that carry pathogens, e.g. mosquitoes)
Reducing spread: hygiene (hand washing), isolating infected people, destroying vectors, and vaccination.
The body's defences
First line — non-specific barriers
- Skin – a physical barrier; also produces antimicrobial substances.
- Nose – hairs and mucus trap particles.
- Trachea and bronchi – mucus traps pathogens; cilia waft it away.
- Stomach – hydrochloric acid kills most pathogens in food.
Second line — white blood cells
- Phagocytosis – white blood cells engulf and digest pathogens.
- Antibody production – white blood cells (lymphocytes) produce antibodies with a shape specific to a pathogen's antigens, marking them for destruction.
- Antitoxins – neutralise toxins released by bacteria.
Immunity and vaccination
After infection, some white blood cells become memory cells. If the same pathogen returns, they produce the correct antibody faster and in greater quantity, so you don't get ill — this is immunity.
Vaccination puts a small, harmless (dead or weakened) form of a pathogen into the body. White blood cells produce antibodies and memory cells without you getting the disease, giving future protection. If enough people are vaccinated, the disease can't spread easily — herd immunity.
Treating disease
- Antibiotics (e.g. penicillin) kill bacteria but do not work on viruses.
- Overuse of antibiotics leads to antibiotic-resistant bacteria (e.g. MRSA) — so they should only be used when necessary.
- Painkillers treat symptoms but don't kill the pathogen.
Worked example
Explain why a vaccine protects you without making you ill.
- It contains a weakened/dead form of the pathogen, so white blood cells make antibodies and memory cells. The pathogen is too weak to cause disease, but if the real one infects you later, memory cells respond quickly and you stay well. ✓
Common mistakes
- Saying antibiotics kill viruses — they only kill bacteria.
- Confusing antigens (markers on the pathogen) with antibodies (made by white blood cells).
- Forgetting that vaccines contain a harmless form of the pathogen.
Exam tips
- Learn one example disease for each of the four pathogen types.
- Explain immunity in terms of memory cells responding faster on second exposure.
- Be able to explain antibiotic resistance and why overuse is a problem.
Key facts to remember
- Pathogens: bacteria, viruses, fungi, protists; spread by contact, water, air/droplets or vectors.
- Defences: barriers (skin, mucus, stomach acid) then white blood cells (phagocytosis, antibodies, antitoxins).
- Vaccination = harmless pathogen → antibodies + memory cells → immunity; antibiotics kill bacteria only (resistance from overuse).