Per the American College of Physicians, an infectious disease (ID) specialist is a doctor of internal medicine (or, in some cases, pediatrics) who is qualified as an expert in the diagnosis and treatment of infectious diseases. After seven or more years of medical school and postgraduate training, ID specialists complete two to three years of additional training in infectious diseases. ID specialists have expertise in infections of the sinuses, heart, brain, lungs, urinary tract, bowel, bones and pelvic organs. Their extensive training focuses on all kinds of infections, including those caused by bacteria, viruses, fungi and parasites. Many ID physicians specialize in treating patients with infections due to human immunodeficiency virus (HIV), the cause of AIDS.