An infectious disease is a disease caused by the presence of pathogenic microorganisms such as viruses, bacteria, fungi or parasites.

Infectious diseases vary in their ability to be spread from one person to another; those that do spread are known as contagious or communicable diseases.

Most minor infectious diseases e.g. warts, respiratory system infections, can be diagnosed by clinical presentation while in other cases it may be necessary to identify the particular microbe.

Many infectious diseases are diagnosed and treated by primary care doctors or by other medical or surgical teams in the hospital. However, in other cases, specialist doctors who have trained and have expertise in infectious diseases will become involved. This may be necessary when e.g. there is not a definite diagnosis, the patient is immunocompromised (their immune system is unable to fight the infection) or the disease has not responded to first line antibiotics.