How does PPEP or Post Exposure Prophylaxis work?

Post Exposure Prophylaxis
is treatment that is given after infection has occurred but before the individual develops symptoms of the disease. PEP can be given for several infections, but PEP for HIV is most well known. I presume this is what you are referring to. PEP works by preventing the infecting organism from multiplying in the body and preventing it from establishing itself. PEP is most effective when it is given within a certain time period after exposure. In the case of HIV, it is most effective if it is started within 72 hours post exposure. PEP for HIV consists of giving a combination of Anti Retroviral drugs, similar to the combination given for treatment for HIV, for a period of 28 days. It works by killing the virus in the body and by preventing it from establishing itself in the immune cells. Even if started early, PEP can occasionally fail but this failure rate is very low. The failure rate increases with increasing time post exposure.