Infection means the invasion of the body by an infecting organism such as a virus or bacterium. Infections can range from minor ailments such as colds to invariably fatal illnesses such as rabies.

Infections may be localized, affecting only a small area, or one system, or they may be generalized, affecting a greater part of the body, as in septicaemia (blood poisoning).

Strictly speaking, a contagious disease is one that is caught by touching an infectious person