A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to quickly produce thousands of identical copies of the original virus. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell.
Virus is not a living organism unlike bacteria or parasites. It can not split itself into two but it rather uses host cells to multiply itself.
While 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, about one hundredth the size of most bacteria.