The smallest known types of infectious agent. It is debatable whether viruses are truly living organisms or just collections of molecules capable of selfreplication under specific conditions. Their sole activity is to invade the cells of other organisms, which they then take over to make copies of themselves. Outside living cells, viruses are inert. A single virus particle (virion) consists of an inner core of nucleic acid, which may be either DNA or RNA, surrounded by 1 or 2 protective protein shells (capsids). Surrounding the outer capsid may be another layer, the viral envelope, which consists mainly of protein. The nucleic acid consists of a string of genes that contain coded instructions for making copies of the virus. Common viral diseases include the common cold, influenza, and chickenpox (caused by the varicella–zoster virus). AIDS is caused by the human immunodeficiency virus ( HIV).


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