A viral disease is an infectious disease due to a virus. The common cold, influenza, chickenpox, measles, and rubella are common in childhood, while lassa fever and yellow fever are important tropical diseases. Viruses may also cause specific organ disease such as hepatitis, meningitis, encephalitis, myocarditis, and pericarditis. Most virus diseases are self-limited and mild, but there are few specific drugs effective in cases of severe illness. Prevention by vaccination is therefore crucial.