Diseases caused by
viruses. Long-term
immunity usually follows viral childhood diseases (
see chickenpox). The common
cold recurs into adulthood because many different viruses cause its symptoms, and immunity against one does not protect against others. Some viruses mutate fast enough to reinfect people after recovery (
see influenza) or to keep the
immune system from fighting them off (
see AIDS). Certain
cancers are caused by viruses.
Vaccines can prevent some viral diseases. Most recently developed antiviral drugs work only against specific viruses; antibiotics are ineffective against viral diseases.
See also poliomyelitis;
smallpox.
For more information on viral diseases, visit Britannica.com.