Describing a drug or other agent that kills or inhibits viruses and is used to combat viral infections. Several types of antiviral drug are now in use, such as aciclovir, effective against herpesviruses, and zidovudine (AZT), a reverse transcriptase inhibitor that is used to treat HIV infection. Protease inhibitors (such as ritonavir and saquinavir), integrase inhibitors (such as raltegravir), and chemokine receptor antagonists (such as maraviroc) are additional classes of antiviral drugs used to treat HIV infection. The body’s own natural antiviral agents, interferons, can now be produced by genetic engineering and are sometimes used therapeutically. However, many antiviral agents are extremely toxic, and viruses evolve rapidly so that a drug’s effectiveness can soon be lost.