A group of infective agents which are so small that they are able to pass through the pores of collodion filters. They are responsible for some of the most devastating diseases affecting humans: for example, INFLUENZA, POLIOMYELITIS, SMALLPOX and YELLOW FEVER. The virus of influenza measures 80 nanometres, whereas the STAPHYLOCOCCUS measures 1,000 nanometres (1 nanometre = one thousand-millionth of a metre).

A single virus particle, known as a virion, comprises an inner core of NUCLEIC ACID which is surrounded by one or two protective coverings (capsid) made of protein. Sometimes the capsid is enclosed by another layer called the viral envelope (also a protein structure). The envelope often disintegrates when the virus invades a cell. Viruses enter cells and then indulge in a complex and variable process of replication using some of the cells’ own structure. Viruses may stay in a host's nucleus, being reactivated months or years later. There are more than a score of large families of viruses, from papillomaviruses, which cause WARTS, through HERPES viruses (cold sores, CHICKENPOX, SHINGLES) and myxoviruses (influenza), to coronaviruses (common cold) and retroviruses (AIDS/HIV). Viral diseases are more difficult to treat than those caused by bacteria: ANTIBIOTICS are ineffective and there are relatively few effective antiviral drugs apart from ACICLOVIR and its analogues, used for Herpes infections. IMMUNISATION is the most effective way of combating viral infections; smallpox, poliomyelitis, MUMPS, MEASLES and RUBELLA are examples of viral diseases which have been successfully combated.