antiviral
An antiviral is a drug that blocks a virus from replicating or from getting into our cells, helping the body bring an infection under control. Because viruses are not alive in the way bacteria are — they hijack our own cells to copy themselves — antiviral drugs face a much harder targeting problem than antibiotics.
Antivirals work by jamming a specific step in the viral life cycle: stopping the virus entering or fusing with a cell, blocking the viral polymerase that copies its genome, inhibiting the viral protease that matures new particles, or preventing finished viruses from being released. To stay safe, they must hit a viral enzyme or step that differs enough from anything in our own cells.
Most antivirals are virus-specific rather than broad — a hepatitis C drug does nothing against influenza — because each virus uses different machinery. They are also prone to resistance, since viruses mutate fast, which is why serious infections like HIV are treated with combinations of antivirals attacking several steps at once so the virus cannot escape all of them.
Sofosbuvir, a polymerase inhibitor, helped turn hepatitis C from a chronic, often fatal disease into one that combination antivirals can cure in most patients.
Combination antivirals can outpace a fast-mutating virus.
Many antivirals are prodrugs — for example nucleoside analogues that must be phosphorylated inside infected cells before they can block the viral polymerase.