After a doctor completes medical school and is licensed to practice medicine, the physician may pursue a rigorous 3 year training program in Internal Medicine. After passing an examination and completing the requirements, the doctor is eligible to become Board Certified in Internal Medicine, whereupon he or she becomes an Internist. This is different from an intern, who is a doctor who has completed medical school and is in the first year of postgraduate training.
Infectious disease specialists (ID) complete two to three additional years of training in diagnosis and treatment of infectious diseases after completing internal medicine training. They specialize in the use of anti-infection drugs and their adverse effects. ID doctors are experts in infection control and sometimes in immunology.