In microbiology, antiviral agents are "agents used in the prophylaxis or therapy of virus diseases. Some of the ways they may act include preventing viral replication by inhibiting viral DNA polymerase; binding to specific cell-surface receptors and inhibiting viral penetration or uncoating; inhibiting viral protein synthesis; or blocking late stages of virus assembly". They tend to be quite specific in effect, and there is far less coverage of viruses by antiviral agents than there is of bacteria by antibiotics.
The widest range of antiviral agents probably is that of drugs targeted against human immunodeficiency virus. There are antiviral agents for diseases that are opportunistic infections in the presence of HIV, such as cytomegalovirus.