The drug enters your cells in an inactive form. It stays completely harmless to normal, healthy host cells because it requires a specific viral enzyme called thymidine kinase to become active. This enzyme is only present inside cells that are actively infected with a herpes virus. 

Once the virus’s enzyme activates the drug, your own cellular enzymes add more components to transform it into its fully lethal form: acyclovir triphosphate. This active compound accumulates in concentrations up to 100 times higher inside virus-infected cells than in healthy cells.