A virus is a microscopic organism that causes infection and disease. Viral illnesses can range from mild to severe, including examples such as the common cold, West Nile virus, polio, and smallpox.
Primary Medical Reviewer
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer
Christine Hahn, MD - Epidemiology
July 8, 2010
eMedicineHealth Medical Reference from Healthwise