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. Koch's postulates are the basis of categorizing diseases as infectious.
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.
Usage as a specialty
An infectious disease physician is an internist who has specialized in the study and treatment of infectious diseases. The specialty can be considered the intersection of internal medicine and microbiology. Such a physician focuses on the disease in individuals, and not necessarily the disease in a larger population.