Viruses are unable to metabolize (break down) food in order to release energy (perform respiration) or proliferate. Viruses can only reproduce (copy themselves), but they require the assistance of a live cell to do so. The host cell is the live cell in which a virus replicates. Virus replication results in the production of more viruses. Viruses depend on cells for reproduction because their manufacture involves many complex biological processes that normal cells can perform more efficiently than viruses. A virus must contain its genetic material (DNA or RNA) inside its protein shell (capsid). In order to infect new cells, it is necessary for the virus to dissolve this protective covering and leave the DNA behind. Normal cells possess enzymes that break down these proteins, allowing them to be absorbed into the cell. Once inside the cell, the DNA begins to replicate, producing more of itself and more copies of the capsid protein. These newly made viruses will then find other cells in which to infect. Some viruses take advantage of the cellular machinery for making proteins. They may be able to use these mechanisms to make more viruses too. This is called viral infection or viremia and can lead to serious health problems if the body's immune system is not able to fight off infected cells. Other viruses simply kill cells in which they replicate by either destroying the cell's ability to function or using the cell's resources to make more viruses.