Viruses can exist in either extracellular or intracellular forms. In its extracellular form, a virus is a microscopic particle containing nucleic acid surrounded by a protein coat and sometimes, depending on the specific virus, other macromolecules. the virus particle, or virion, is metabolically inert and cannot generate energy or carry out biosynthesis. the virus genome moves from the cell in which it was produced to another cell inside the virion. Once in the new cell, the intracellular state begins and the virus replicates. New copies of the virus genome are produced, and the components of the virus coat are synthesized. Certain animal viruses (such as polio and respiratory syncytial virus) may skip the extracellular stage when moving from cell to cell within the same organism. Instead, they mediate the fusion of infected cell with uninfected cells and transfer themselves in this way. However, when moving from one organism to another they are truly extracellular.