Pre-exposure prophylaxis (PrEP) is a relatively new treatment for at-risk patients that can reduce the chances of an individual contracting HIV by more than 90 percent. However, it should not be seen as a vaccine. Dr. Claudia Goulston, associate professor of infectious diseases and a specialist in HIV, explains what exactly PrEP is, who it is good for and some important things you need to know about the treatment before you consider starting it.