Viruses exist at the boundary between life and non-life. They consist of nucleic acids and proteins but cannot reproduce without infecting a host cell. Viruses infect and take over the machinery of cells to produce more viruses. Once inside a host cell, a virus will either enter a lytic cycle that destroys the cell to release new viruses, or a lysogenic cycle where the viral DNA integrates with the host DNA in a dormant, parasitic relationship. Viruses are highly specialized to infect specific host cells and hijack their functions to copy themselves.