Physicians diagnose diseases and injuries, administer treatment, and advise patients on good diet and other ways to stay healthy. The United States has two kinds of physicians, the Doctor of Medicine (MD) and the Doctor of Osteopathy (DO). Both use medicines, surgery, and other standard methods of treating disease. DOs place special emphasis on problems involving the musculoskeletal system, which includes muscles, ligaments, bones, and joints.

Patients receive medical care from primary care doctors and specialists. Primary care doctors include general practitioners, family physicians, general internists, and general pediatricians. Many women also use obstetricians-gynecologists as primary care doctors. Patients usually consult a primary care doctor when they first become ill or injured. Primary care physicians can treat most common disorders, and provide comprehensive, lifelong care for individuals and families.

But medical knowledge has advanced so far that no physician can master an entire field of medicine. Primary care doctors may refer patients with unusually complicated problems to specialists with advanced training in a particular disease or field of medicine. Specialists may even concentrate in one particular area, and become subspecialists. Each specialist in internal medicine, for instance, is an expert in diagnosis and nonsurgical treatment of adult diseases. But some internists take advanced training to become subspecialists in treating adolescents, heart disease, elderly people, cancer, or arthritis. For more information about the areas that specialists treat, see the table on Medical Specialties.