A virus is a microscopic organism that causes infection and disease. Viral illnesses can range from mild to severe. Some examples are the common cold, West Nile virus, polio, and smallpox.
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerChristine Hahn, MD - Epidemiology
Current as ofSeptember 9, 2014
WebMD Medical Reference from Healthwise