Infection is a harmful disease caused by microorganisms (like viruses, bacteria, fungi and parasites) that enter the body and multiply. Viruses, bacteria and fungi can cause infections in various parts of the body, including the skin, lungs, gut and heart. The germs may produce toxins that harm tissues and make you sick. Or they may pass from person to person in the form of droplets or particles, such as those spread when a person coughs or sneezes. Infections that spread this way are called airborne or droplet transmission, and examples include influenza, measles and chickenpox. Germs can also spread through direct contact with an infected person, such as touching a sore or contaminated finger. They can also spread by sharing personal items, such as toothbrushes, combs, razor blades and cutlery. Examples of infectious diseases spread this way are scabies, head lice, ringworm and genital infections (like gonorrhoea or STIs).
Other types of infection, like protozoan infections such as malaria and giardiasis, and helminth infections such as pinworm and roundworm, are spread by insects that enter the body through bites. Occasionally, these microorganisms can spread from animal to human through contact with fur, feathers and scales, as well as contaminated food, water or surfaces.
Globalization and travel increase the risk of spreading infectious diseases through a number of different routes. This is partly due to changes in human behavior, such as sex, substance misuse and intravenous drug use, and it is also because the time it takes for an infection to develop can be shorter than the travel times of germs.