Internal medicine specialists, diagnose and treat diseases in adults. An internist may act as your primary care physician, treating you for general illnesses as well as providing complex care. They are trained in the diagnosis and treatment of cancer, infections, and diseases affecting the heart, blood, kidneys, joints, and the digestive, respiratory, and vascular systems. A general internist handles the broad, comprehensive spectrum of illnesses that affect adults. General internists are recognized as experts in diagnosis, in treatment of chronic illness, and in health promotion and disease prevention they are not limited to one type of medical problem or organ system.