Infection and response
Biology · GCSE · The School
Four kinds of pathogen
BACTERIA are living cells that divide rapidly and often damage tissue by producing toxins. VIRUSES are not cells; they enter a host cell, use it to make copies, and burst it. PROTISTS are single-celled organisms, often carried by a vector such as a mosquito. FUNGI grow into tissue. Which one causes an illness decides what can treat it.
How the body defends itself
First the barriers: skin, mucus, cilia, stomach acid, tears. If a pathogen gets past, white blood cells act in three ways — engulfing it (phagocytosis), producing ANTIBODIES that match its surface antigens, and producing antitoxins. The antibody response is SPECIFIC: it recognises that pathogen and no other, which is why the second infection is usually milder.
Vaccination and antibiotics
A vaccine introduces a dead or inactive form so the immune system produces antibodies and MEMORY cells without the illness. Antibiotics kill bacteria; they do nothing to viruses, so taking them for a cold treats nothing and helps resistant bacteria survive. RESISTANCE arises because the few bacteria that survive a course reproduce, and their offspring inherit the resistance.
Measles is a VIRUS — antibiotics do nothing; prevention is by vaccination. Salmonella is a BACTERIUM — antibiotics work; prevention is hygiene and cooking. Rose black spot is a FUNGUS — treat with fungicide and remove affected leaves. Malaria is caused by a PROTIST spread by mosquitoes — control the vector with nets and drainage. The treatment follows the type, which is why identifying the type comes first.
Why antibiotic resistance is not the bacteria adapting
The wrong version of this costs marks every year: bacteria do not become resistant because they were exposed to the drug. Resistance arises by random mutation in a few individuals BEFORE the antibiotic arrives. The antibiotic then kills the non-resistant majority, leaving the resistant ones to reproduce without competition — so the population becomes resistant while no individual bacterium ever changed. Say mutation first, selection second. This is also why finishing a course matters: stopping early leaves the least susceptible survivors alive to multiply.
Herd immunity, stated so it earns the mark
Vaccination protects the person vaccinated, and above a threshold proportion of the population it also protects those who are not — because an infected person is unlikely to meet a susceptible one before recovering, so transmission chains break. Say that mechanism, not just that 'others are protected too'. The threshold differs by disease according to how easily it spreads, and this is why people who cannot be vaccinated, such as those with certain immune conditions, depend on the coverage around them. A question asking why vaccination is offered to a whole population is asking for exactly this.