Viruses are intracellular parasites and depend on the host cells for their food, growth and multiplication. Chemotherapy can interfere with any of the steps involved in viral replication. Acyclovir inhibits viral DNA synthesis and it is effective against Herpes simplex virus type 1 and type 2, varicella zoster virus and Epstein-Barr virus. Ganciclovir, penciclovir, cidofovir are also effective against herpes viruses. Amantadine and rimantadine inhibit the replication of influenza a virus and may be used during an epidemic. Oseltamivir and zanamivir are indicated in the prevention and treatment of swine flu. Drugs used in the treatment of AIDS are of two groups- reverse transcriptase inhibitors and protease inhibitors. Combination of drugs (HAART) is used in AIDS to improve prognosis. Zidovudine is a reverse transcriptase inhibitor active against HIV and other retroviruses. Didanosine, zalcitabine, stavudine, lamivudine and abacavir are other reverse transcriptase inhibitors used as alternatives to azidothymidine in patients with advanced HIV infection. NNRT inhibitors are effective only against HIV-1. Nevirapine is used in pregnant women and in the new born to prevent vertical transmission to the new born. Protease inhibitors are used in combination with other antiretroviral drugs. Various drug interactions can occur with the use of antiretroviral drugs.