What if there was a pill you could take once a day that would prevent the transmission of HIV. Would you take it? Well there is a pill that has proven 90% (some results suggest upto 99%) effective against HIV transmission and its called Truvada. But how does it work? “PrEP” stands for Pre-Exposure Prophylaxis. The word “prophylaxis” means to prevent or control the spread of an infection or disease.