A protease inhibitor is a chemical compound that inhibits the functions of proteases to control viral infections, especially so for the treatment of HIV/AIDS and hepatitis C, but also for the treatment of other viruses, such as chicken pox. All cells have protease enzymes that cleave cellular proteins, which are required for normal cell life cycles. By distrupting the use of natural proteases within the cell, the use of protease inhibitors causes severe side effects, and because the half-life of many protease inhibitors is short (3-5 hr.), the drugs must be taken every 4-6 hours. Protease inhibitors are often structural mimics of protein active (binding) sites on which the proteases would normally bind.