Infectious disease

An infectious disease is caused by a pathogenic living organism. While all contagious diseases are infectious, not all infectious diseases are contagious from a host of one species to a host of another.

Contagion may need to occur through an intermediate vector (biological): unless there is direct blood-to-blood contact, for example, malaria is not contagious between humans. Malaria is transmitted when a mosquito bites an infected host, and, in biting an uninfected host, transfers infected blood.