A virus is a small infectious agent that replicates inside the living cells of organisms. It is a small infectious agent that can replicate only inside the living cells of organisms. Most are too small to be seen by anything but a microscope and they infect all organisms from animals to plants to bacteria. The first one discovered was the tobacco virus in 1898. Since then around 5,000 viruses have been described in detail although there are millions of different types. They are found in nearly every ecosystem on Earth and are the most abundant type of biological entity. Virology is the study of viruses.
They consist of two to three parts: the genetic material made from either DNA or RNA, molecules that carry genetic information, a protein coat that protects these genes; and in some cases an envelope of lipids that surround the protein coat.

They spread in various ways. Plant viruses are spread from insects that feed on sap while animal viruses may be spread through blood-sucking insects. These organisms are known as vectors. Influenza is spread through coughing and sneezing. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. Viruses can only infect a certain range of hosts called the “host range”. It can be narrow or broad when capable of infecting many species.

Most animals have an immune response that eliminates the infecting virus. Vaccines also can produce immune responses. AIDS, however, can evade immune responses and result in chronic infections. Antibiotics have no effect on viruses.

In the 19th century viruses were defined in terms of their infectivity, their ability to be filtered, and their requirements for a living host. Soon after came the need to grow viruses for their vaccines. In 1931, Ernest William Goodpasture grew influenza and several other viruses in fertilized chicken’ eggs. In 1949 polio virus was cultured human embryo cells and it was the first virus to be grown without using solid animal tissue or eggs.

In 1931, after the electron microscopy was invented the first images of viruses were obtained. Wendell Meredith Stanley examined the tobacco Mosaic virus and found it was mostly made of protein. It was the first to be crystallized and its structure was then studied in detail. Most of the 2,000 recognized species of animal, plant, and bacterial viruses were discovered in the second half of the 20th century.

They come in a wide display of shapes and sizes called morphologies. They are generally smaller than bacteria. The whole virus particle is known as a virion. There is also a large variety in genomic structures of viruses, in fact, they contain more structural genomic diversity than plants, animals, archaea, or bacteria.

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