• virus
  • Submicroscopic agents that infect plants, animals and bacteria, and are unable to reproduce outside the tissues of the host. A fully formed virus consists of nucleic acid (DNA or RNA) surrounded by a protein and lipid (fat) coat. The nucleic acid of the virus interferes with nucleic acid-synthesizing mechanism of the host cell, organizing it to produce more viral nucleic acid. Viruses cause many diseases (e.g., mosaic diseases of many cultivated plants, myxomatosis, foot and mouth disease, the common cold, influenza, measles, poliomyelitis). Many plant viruses are transmitted by insects, some by eelworms. Animal viruses are spread by contact, droplet infection or by insect vectors and some are spread by the exchange of body fluids.
Abstract from DBPedia
    A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses 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 virus by Martinus Beijerinck in 1898, more than 9,000 virus species have been described in detail of the millions of types of viruses in the environment. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. The study of viruses 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 virus. When not inside an infected cell or in the process of infecting a cell, viruses 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 virus 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 virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of viruses 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, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Viruses 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, viruses have been described as "organisms at the edge of life", and as replicators. Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Many viruses, including influenza viruses, 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 viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its "host range". This can be narrow, meaning a virus 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 virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, 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.

    ウイルス(英語: virus〔ヴァイラス〕, ラテン語: virus〔ウィールス〕, 中国語: 病毒)は、他生物の細胞を利用して自己を複製させる、極微小な感染性の構造体で、タンパク質の殻とその内部に入っている核酸からなる。生命の最小単位である細胞やその生体膜である細胞膜も持たないこと、小器官がないこと、自己増殖することがないことから、生物かどうかについて議論がある。 一般的には「ウイルスは生物ではない」とされるが、フランスの進化生物学者パトリック・フォルテールのように、生物に含める見解もある。ウイルスが宿主に感染した状態(ヴァイロセル、virocell)を本来の姿と捉えれば生物のようにふるまっていること、ミミウイルスのように多数の遺伝子を持った巨大なウイルスもあることなどを理由としている。 ウイルスを生命体と見なせば、その数や多様性は地球上で最も多く(みなさない場合、個体数は微生物、種類は甲虫類が最も多い)、メタゲノム解析の実用化により様々な環境にウイルスが見つかっている。宿主に残ったウイルス由来の遺伝子が生物進化に関わったり、地球の生態系や気候にも影響を与えたりしている。動物や植物のほかほぼ全ての生物に特有のウイルスが存在する。ヒトを含めた動植物に感染症など疾病を引き起こすウイルスは一部であるが、発見・分析されていないウイルスが野生鳥獣を宿主とするものだけで170万種あり、その半数が人獣共通感染症の病原体になるリスクがあると推計されている。