Viruses consist of only a protein coat and either DNA or RNA. There is currently debate as to whether to include them with living things; as of now, they are not, for several reasons. Viruses do not use energy and they are not cells. More pertinent to your question is that they cannot reproduce on their own. Viruses "hijack" the reproductive apparatus of living cells. The protein coat attaches to the outside of a cell and injects its own DNA or RNA into the cell. The viral genetic material "takes over" the cells DNA. Instead of the cell producing all of the things it needs to survive and make copies of itself, the viral DNA/RNA uses the cell to produce multiple new complete viruses. When the cell can no longer contain the new viruses, it lyses (ruptures) or buds, releasing many new viruses to repeat the process.