what is a virus?

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Virus diseases inflict a heavy illness and economic burden on humans and animals and can devastate agricultural crops. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. Their shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Viruses infect cellular forms of life and are grouped into animal, plant and bacterial viruses.
It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection. They have been described as organisms at the edge of life. Viral infections in human as well as animal hosts, usually result in an immune response and disease. Often, a virus is completely eliminated by the immune system. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent virus infections.