A virus is a sub-microscopic infectious agent, which can only replicate inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. Only around 5,000 virus species have been described in detail out of the estimated millions of types of viruses present in the environment. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.

 While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of:

  1. The genetic material, which can either be DNA or RNA coding for the structural proteins of the viral assembly;
  2. A protein coat, the capsid, which surrounds and protects the genetic material; and
  3. An outside envelope of lipids but this is not found for all viruses.

 The shapes of the virions range from simple helical and icosahedral forms to more complex structures. Most virus species cannot be observed with an optical microscope, only measuring about 1/100 the size of most bacteria.

 Viruses are able to spread in many different ways. For example, the influenza virus is spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted via the mouth, passed by contact and entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. Viral infections in animals provoke an immune response which usually eliminates the infecting virus. Some specific viruses, including those that cause AIDS, HPV infection, and viral hepatitis, have developed mechanisms to evade these immune responses and thus result in chronic infections. Several antiviral drugs have been developed and intensive research into new antiviral drugs is ongoing.