Diseases caused by virus es.
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 cancer s are caused by viruses. Vaccine s 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 .