Foscarnet is active in vitro (test tube) against herpes viruses, including CMV, by inhibiting the virus DNA polymerases, enzymes necessary for virus replication, without affecting cellular DNA polymerases. Opportunistic CMV disease in AIDS is usually seen as retinitis, colitis, esophagitis, hepatitis, pancreatitis, encephalitis, or pneumonia. Ganciclovir has been used to treat AIDS patients with CMV disease but can cause severe neutropenia (very low neutrophil cell counts). Foscarnet does not suppress the production of neutrophils or other leukocytes (myelosuppression) and has shown in vitro activity against HIV.