Every day your body is exposed to millions of microorganisms, yet you stay healthy most of the time. This is because you have a defence system that keeps harmful invaders out and destroys those that get in. In this topic you will learn what causes disease, how diseases spread, and the clever ways — from your skin to white blood cells to vaccines — that your body fights back.
A pathogen is a disease-causing organism. Pathogens include bacteria, viruses, fungi and protoctists. A transmissible disease (also called a communicable disease) is one in which the pathogen can be passed from one host to another.
Cholera is a useful example: the bacterium is taken in through contaminated water, multiplies in the small intestine and produces a toxin. This causes water and salts to leave the blood and tissues into the gut, leading to severe diarrhoea, dehydration and loss of ions.
Because pathogens move between hosts, breaking that pathway reduces disease. The body itself helps, but so do hygiene and public-health measures.
The skin and other surfaces also act as a mechanical barrier, so keeping them clean and unbroken is an important first line of defence.
Your body has both mechanical and chemical barriers, plus the action of white blood cells.
If pathogens get past these barriers, white blood cells defend the body. Phagocytes engulf and digest pathogens (phagocytosis). Lymphocytes produce antibodies — proteins that lock onto specific antigens on a pathogen, marking it for destruction or clumping pathogens together.
Each pathogen has its own antigens, so antibodies are specific — one type fits one antigen. After an infection, some lymphocytes remain as memory cells, giving long-term protection.
If many people are vaccinated, there are fewer hosts for the pathogen, so it cannot spread easily — this protects even the unvaccinated and can control epidemics.
Practise exam-style questions on this topic.