A virus is a small pathogen that can replicate only inside the living cells of organisms. Most viruses are too small to be seen directly with a optical microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since the initial discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology.
Virus particles (known as virions) consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an viral envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helix and icosahedron forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium.
The origins of viruses in the evolutionary history of life are unclear: some may have evolution from plasmids – pieces of DNA that can move between cells – while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.
Viruses spread in many ways; 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 Hematophagy insects. These disease-bearing organisms are known as Vector (epidemiology). influenza are spread by coughing and sneezing. The norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the fecal-oral route and are passed from person to person by contact, entering the body in food or water. HIV is one of several viruses transmitted through Sexual intercourse and by exposure to infected blood. Viruses can infect only a limited range of host cells called the "Host (biology)#Host_range". This can be narrow or, as when a virus is capable of infecting many species, broad.
Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an immunity (medical) to the specific viral infection. However, some viruses including those causing AIDS and viral hepatitis evade these immune responses and result in Chronic (medical) infections. Antibiotics have no effect on viruses, but several antiviral drugs have been developed.