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I dsDNA viruses
II ssDNA viruses
III dsRNA viruses
IV (+)ssRNA viruses
V (-)ssRNA viruses
VI ssRNA-RT viruses
VII dsDNA-RT viruses 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. Viruses infect all cellular life. The first known virus, tobacco mosaic virus, was discovered by Martinus Beijerinck in 1899,[1] and now more than 5,000 types of virus have been described.[2] The study of viruses is known as virology, and is a branch of microbiology. Viruses consist of two or three parts all viruses have genes made from either DNA or RNA, long molecules that carry genetic information; all have a protein coat that protects these genes; and some have an envelope of fat that surrounds them when they are outside a cell. Viruses vary in shape from simple helical and icosahedral shapes, to more complex structures. They are about 100 times smaller than bacteria.[3] 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. Viruses spread in many ways; different species of virus use different methods. For example, plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food or water. Rotavirus is often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sex.
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