Immunization has proved to be the most effective way of controlling viral diseases, and the development of antiviral drugs has lagged behind the development of antibacterial agents. Many of the early antiviral agents were DNA nucleoside analogs such as idoxuridine, cytarabine, and vidarabine, which competitively inhibited replication of DNA viruses, especially herpes viruses. An important advance was the deliberate synthesis of a purine nucleoside analog, acyclovir, which is only active following phosphorylation, which is carried out selectively by virus-coded thymidine kinase. Acyclovir is active against some herpesviruses, particularly herpes simplex virus, but not against RNA viruses, and appears to have low toxicity because of the low level of phosphorylation by host cell thymidine kinase.