The virus is a small infectious factor that replicates only in living cells of other organisms. Viruses can infect all kinds of life forms, from plants to microbes, including bacteria and archaea.

Since Dmitri ivanovsky published an article describing the infection of tobacco plants by non bacterial pathogens in 1892, and Martinus Beijerinck discovered tobacco mosaic virus in 1898, about 5000 viruses have been described in detail, although there are millions of them. [4] Viruses exist in almost every ecosystem on earth and are the most abundant types of biological entities. Virus research is called virology, which is a sub specialty of microbiology.

Although viruses are not in the infected cells or in the process of infecting cells, they exist as independent particles. These virus particles, also known as virus particles, consist of two or three parts:

Viruses are transmitted in many ways; viruses in plants are usually transmitted between plants by insects that feed on plant juices, such as aphids. Blood sucking insects can carry animal viruses. These organisms with diseases are called vectors. The flu virus is spread by coughing and sneezing. Norovirus and rotavirus are common causes of viral gastroenteritis. They are transmitted through the fecal oral route, between people through contact, and into the human body through food or water. HIV is one of several viruses transmitted through sexual contact and contact with infected blood. The range of host cells that a virus can infect is called "host range.". This can be narrow, meaning the virus can infect a few species, or it can be broad, meaning it can infect many species.

Virus infection in animals can cause an immune response, usually eliminating the infected virus. Vaccines can also produce immune responses that confer artificially acquired immunity against specific viral infections. Some viruses, including those that cause AIDS and viral hepatitis, escape these immune responses and lead to chronic infections. Antibiotics have no effect on the virus, but several antiviral drugs have been developed.