<P> A virus is a biological agent that reproduces inside the cells of living hosts . When infected by a virus, a host cell is forced to produce thousands of identical copies of the original virus at an extraordinary rate . Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell . But unlike still simpler infectious agents, viruses contain genes, which gives them the ability to mutate and evolve . Over 5,000 species of viruses have been discovered . </P> <P> The origins of viruses are unclear: some may have evolved from plasmids--pieces of DNA that can move between cells--while others may have evolved from bacteria . A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds the protein coat and is used, in combination with specific receptors, to enter a new host cell . Viruses vary in shape from the simple helical and icosahedral to more complex structures . Viruses range in size from 20 to 300 nanometres; it would take 33,000 to 500,000 of them, side by side, to stretch to 1 centimetre (0.39 in). </P> <P> Viruses spread in many ways . Just as many viruses are very specific as to which host species or tissue they attack, each species of virus relies on a particular method for propagation . Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors . Some viruses of animals, including humans, are spread by exposure to infected bodily fluids . Viruses such as influenza are spread through the air by droplets of moisture when people cough or sneeze . Viruses such as norovirus are transmitted by the faecal--oral route, which involves the contamination of hands, food and water . Rotavirus is often spread by direct contact with infected children . The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex . Others, such as the Dengue virus, are spread by blood - sucking insects . </P> <P> Viral infections can cause disease in humans, animals and even plants . However, they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus . Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life - threatening infections . Vaccines that produce lifelong immunity can prevent some viral infections . </P>

Importance of knowing the structure of viruses in virology