Internal medicine or general medicine (in Commonwealth nations) is the medical specialty dealing with the prevention, diagnosis, and treatment of adult diseases. Physicians specializing in internal medicine are called internists, or physicians (without a modifier) in Commonwealth nations.Internists are skilled in the management of patients who have undifferentiated or multi-system disease processes.he term internal medicine originates from the German term Innere Medizin, popularized in Germany in the late 19th century to describe physicians who combined the science of the laboratory with the care of patients.