Viruses are parasites of cells. A typical virus penetrates a cell, commandeers its metabolic machinery, assembles hundreds of new viruses that are copies of itself, then leaves the cell to infect other cells. In the process, the host cell is usually destroyed.
Viruses are specific for the kinds of cells they will parasitize. Some viruses will attack only one kind of cell within a single host species, while others will attack similar cells from a range of closely related species. Bacteriophages, or phages, for example, are viruses that attack only bacteria.
Viruses consist of a nucleic acid surrounded by a protein coat called a capsid. Viruses are categorized by the kind of nucleic acid they contain; that is, they are either DNA viruses or RNA viruses. The capsids of some viruses include an envelope that assists them in penetrating their hosts. Envelopes incorporate phospholipids and proteins obtained from the cell membrane of the host.