Infectious disease medicine is the subspecialty of internal medicine that focuses on diagnosing and managing infections. Although most common infections are treated by general internists and other specialty physicians, internists practicing infectious disease medicine are frequently called upon to help diagnose unknown infections and assist in managing difficult, unusual, or complicated infections. Infectious disease medicine requires an extensive understanding of the way in which bacterial, viral, fungal, and parasitic infections occur in humans and how they present clinically, as well as knowledge about antimicrobial agents, antibiotic resistance, vaccines, and other immunobiological agents. Because of their training, infectious disease internists are also uniquely equipped to deal with the environmental, occupational, and host factors that predispose to infection, as well as the basic principles of epidemiology and transmission of infection.