Antiviral refers to a substance or agent that is effective against viruses. It can be a drug, vaccine or other substance that can prevent, inhibit or treat viral infections. Antivirals are commonly used to treat infections caused by viruses such as HIV, hepatitis B and C, influenza, herpes, and others. They work by targeting specific components of the virus, such as its ability to replicate or enter cells, thereby preventing the virus from spreading and infecting new cells.