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, or a lysogenic cycle that establishes a long-term parasitic relationship with the cell. Viruses are found everywhere and are responsible for many diseases in humans and other organisms.