Infectious disease is the subspecialty of internal medicine dealing with the diagnosis and treatment of communicable diseases of all types, in all organs, and in all ages of patients. The specialty requires an understanding of the microbiology, prevention, and management of disorders caused by viral, bacterial, fungal, and parasitic infections, including the appropriate use of antimicrobial agents, vaccines, and other immunobiological agents; the environmental, occupational, and host factors that predispose to infection; and the basic principles of epidemiology and transmission of infection. The emergence of drug-resistant bacteria and the AIDS epidemic have significantly affected this specialty.
Infectious disease physicians traditionally worked in university medical centers and had research backgrounds tracking the source of infection in outbreaks of disease. Today, they are experts in preventive medicine and travel-related conditions as well as having moved to a more patient-oriented practice. They often see referrals with serious infections or problematic responses to treatment. Practice in the field requires a large physician referral base, and some infectious disease physicians practice general internal medicine as well.