Antiviral drug

An antiviral drug is a medicine used to treat or prevent infection by a virus. Viruses are hard to target because they multiply inside the host's own cells using the cell's machinery, so a useful drug must hit something that belongs to the virus, such as one of its enzymes, while sparing the cell. Unlike a vaccine, which prepares the immune system in advance, an antiviral acts on the virus during an infection; antibiotics have no effect on viruses.

Each class blocks a different stage of the viral life cycle. Entry and fusion inhibitors stop a virus attaching to or getting into the cell. Many drugs are nucleoside analogs, fake building blocks that stop the viral genome being copied; acyclovir, a look-alike of guanosine switched on by a herpesvirus enzyme, ends the growing DNA chain. HIV is treated with reverse transcriptase, integrase and protease inhibitors, and hepatitis C with drugs that block its polymerase and protease. Neuraminidase inhibitors such as oseltamivir keep new influenza particles from leaving the cell. Most antivirals work against only one virus or one family.

Viruses mutate quickly, so resistance is a constant problem, which is why HIV is treated with several drugs at once. Gertrude Elion's work leading to acyclovir, the first selective drug against herpesviruses, was part of a 1988 Nobel Prize.