The viruses, unlike most bacteria, are obligate intracellular pathogens that use biosynthesis mechanisms and enzymes of the host cells for replication. Hence, it was feared that it may not be possible to inhibit viral replication without also being toxic to host cells, but currently newer antiviral drugs are used successfully for treatment of few viral diseases without causing much of toxicity or serious side effects.
The first antiviral drugs to be used were like selective poisons that targeted cells with intensive DNA and RNA synthesis. Recently used antiviral drugs, however, act specifically against virus-coded enzymes or structures of the virus that are impor-tant for replication of the viruses.
Marboran was the first antiviral drug used clinically for effective treatment of poxvirus infection in 1960. The compound was used successfully against eczema vaccina-tum and smallpox. Subsequently in 1962, an antineoplastic agent idoxuridine was found to be effective for treatment of herpetic eye infection, and amantadine (a molecule with an unusual structure) was found effective for treatment of influenza A virus.
In 1970s, acyclovir (ACV) was used most successfully for treatment of herpes virus infection by administer-ing the drug parenterally. Subsequently, many antiviral agents have been distributed for use against many viral infections including human immunodeficiency virus (HIV).