A<unk>  is a submicroscopic infectious agent that replicates only inside the living cells of an organism.<unk> es infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic<unk>  by Martinus Beijerinck in 1898, more than 9,000<unk>  species have been described in detail of the millions of types of<unk> es in the environment.<unk> es are found in almost every ecosystem on Earth and are the most numerous type of biological entity. The study of<unk> es is known as virology, a subspeciality of microbiology. When infected, a host cell is often forced to rapidly produce thousands of copies of the original<unk> . When not inside an infected cell or in the process of infecting a cell,<unk> es exist in the form of independent particles, or virions, consisting of (i) the genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the<unk>  acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these<unk>  particles range from simple helical and icosahedral forms to more complex structures. Most<unk>  species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of<unk> es in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution,<unk> es are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction.<unk> es are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities,<unk> es have been described as "organisms at the edge of life", and as replicators.<unk> es spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example,<unk> es are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and<unk> es in animals can be carried by blood-sucking insects. Many<unk> es, including influenza<unk> es, SARS-CoV-2, chickenpox, smallpox, and measles, spread in the air by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several<unk> es transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a<unk>  can infect is called its "host range". This can be narrow, meaning a<unk>  is capable of infecting few species, or broad, meaning it is capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting<unk> . Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some<unk> es, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed.
Virus
Microorganism
Bacteria
Drug
Drug 
Fungus
Biological agent
Chemical substance