Viral illnesses are among the most common clinical problems encountered in medicine. Some have important epidemiologic implications and some are preventable by immunization, improved sanitation or other means. Recently, drugs have been developed which show promise for specific prophylaxis or treatment. It is thus increasingly important that physicians in practice be acquainted with methods of identification of common viral illnesses so they can intervene with appropriate counsel, preventive measures or treatment for their own patients. Physicians should also consider the larger epidemiologic implications of many viral illnesses and cooperate with local, state and national efforts to accurately determine the number of patients involved and the severity of their illnesses. It is only with this sort of information that an estimate of medical, social and economic impact of the common viral diseases can be made, so that attention will be given to development of better prevention and methods of control.