Acyclovir (Zovirax) and zidovudine (Retrovir) dominate antiviral therapy. They interfere with the multiplication of herpes viruses (acyclovir) and HIV (zidovudine) by incorporation into nascent DNA chains and interruption of the further linking of nucleotides. All types of infection caused by herpes simplex virus are potentially treatable by acyclovir, but treatment has to start to be effective. It is especially important to treat immunosuppressed patients because their infections are more prolonged and severe. A typical attack of herpes zoster in an immunocompetent patient is shortened by about 2 days if high doses of acyclovir are given within 3 days of the start of the skin lesions, but unfortunately the incidence of post-herpetic neuralgia is not diminished. Zidovudine lowers early mortality in patients with AIDS and pneumocystis carinii pneumonia. However, much of the effectiveness of zidovudine is lost later; the average prolongation of life in treated patients is estimated to be about 1 year. Some two thirds of patients with AIDS can be treated with zidovudine; in the others the drug is ineffective (Kaposi's sarcoma) or contraindicated. Frequent blood counts are necessary to monitor myelotoxicity.