An infectious disease is caused by a pathogen, a microorganism that can be passed from one host to another. The main types of pathogen are bacteria (single cells), viruses (tiny particles that reproduce only inside host cells), fungi and protists.

Pathogens spread through the air in droplets, by direct contact, through contaminated food and water, and by vectors such as mosquitoes.

First line: barriers that keep pathogens out, such as skin, mucus and cilia in the airways, tears and stomach acid.

Second line: if pathogens get in, phagocytes (a type of white blood cell) engulf them, and the area becomes inflamed: red, warm and swollen. A fever may develop.

Third line: lymphocytes recognize the antigens (marker molecules) on a particular pathogen and make antibodies that fit those antigens and help destroy the pathogen. Some become memory cells, which respond much faster next time.

Vaccination gives the body a safe version of a pathogen's antigens so that it makes antibodies and memory cells without having the disease. If enough people in a community are immune, the pathogen cannot spread easily: this is herd immunity.

Antibiotics are medicines that kill bacteria or stop them growing. They do not work against viruses.