One of the smallest known types of infectious agent. It is debatable whether viruses are truly living organisms or just collections and gatherings of molecules capable of self-replication under specific conditions. Their sole activity is to invade the cells of other living organisms, which they then take over to make duplicates of themselves. Outside living cells, viruses are inert.
A single virus particle (virion) consists of an inner core of nucleic acid, which can be either RNA or DNA, surrounded by one or two protective protein shells (capsids). Surrounding the outer capsid may be another layer, the viral envelope, which consists mostly of protein. The nucleic acid consists of a string of genes that hold coded instructions for making copies of the viruses and counterparts.
Widespread viral conditions include the common cold, cold sores, chicken pox, influenza, and warts. AIDS is caused by human immunodeficiency virus or HIV.