18.1: Viruses and Bacteria Viruses What is a Virus? Virus: composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium Biologists consider viruses to be nonliving because viruses are not cells. Viruses do not carry out respiration, grow or develop. Viruses can only multiply when they are inside a living cell. Host cell: cell in which a virus replicates Almost all viruses are parasites because they destroy the cells in which they multiply. Viruses can infect the organisms of all six kingdoms. Because viruses are not alive, scientists do not use binomial nomenclature to name them.