Viruses (lat. Virus poison) are non-cellular life forms that have their own genome and are capable of reproducing only in the cells of more highly organized creatures. Viruses as a whole are characterized by two forms of existence: extracellular, or resting, and intracellular, multiplying (reproducing), or vegetative. The terms “virus particle”, “virus particle”, “virion” are also synonymous with the first name, and the virus – cell complex are synonyms of the second.
Viruses exist in nature, transmitted from one host to another. Reproduction of Viruses can occur only intracellularly, and therefore Viruses are obligate intracellular parasites of animals, plants, insects, bacteria, fungi and other classes of living things. The intracellular parasitism of Viruses is due to the fact that, due to the extreme simplicity of their organization, they use the cellular synthetic apparatus (ribosomes, membranes), enzymes and energy-generating systems for their reproduction. The connection between two forms of V.'s existence is carried out through the nucleic acid to the virion, edge induces virus-specific syntheses in the infected cell and, ultimately, the formation of daughter viral particles. Various species of V. at the extracellular stage of existence are characterized by a relatively large size variability (from 15–18 to 3000–3500 nm). The largest complex V. arranged from the genus of poxviruses (Poxvirus), to which the causative agents of smallpox and V. smallpox vaccines belong, are distinguishable in an ordinary light microscope, but most of V. possesses submicroscopic sizes and is distinguishable only in an electron microscope.