Post-exposure prophylaxis (PEP) is any treatment started immediately after exposure to a disease-causing virus, in order to prevent the disease from breaking out.

PEP is commonly used, and very effective, to prevent the outbreak of rabies after a bite by a rabid animal. The treatment consists of repeated injections of rabies vaccine and immunoglobulin.

Post-exposure prophylaxis with a course of antiviral drugs is also thought to reduce the risk of seroconversion after exposure to HIV. To be effective, it must be started as soon as possible after exposure and no later than 72 hours. The treatment for HIV lasts four weeks. While there is compelling data to suggest that PEP after HIV exposure is extremely effective, there have been cases where it has failed. The regime can be very demanding and have unpleasant side effects.