That is one more reason why FDA and other regulatory authorities work to assure that there are multiple safeguards, not just testing. Such medical errors occur very rarely, but given that there are over 20 million transfusions every year, in the USA, they can occur. Collection of blood from persons with an increased risk of HIV infection also presents an added risk if blood were to be accidentally given to a patient in error either before testing is completed or following a positive test. This is important because the rate of new infections in MSM is higher than in the general population and current blood donors. FDA's MSM policy reduces the likelihood that a person would unknowingly donate blood during the "window period" of infection. The "window period" is the time between being infected with HIV and the ability of an HIV test to detect HIV in an infected person. Detection of HIV infection is particularly challenging when very low levels of virus are present in the blood for example during the so-called "window period". Therefore, even a failure rate of 1 in a million can be significant if there is an increased risk of undetected HIV in the blood donor population. While today's highly sensitive tests fail to detect less than one in a million HIV infected donors, it is important to remember that in the US there are over 20 million transfusions of blood, red cell concentrates, plasma or platelets every year. Blood donor testing using current advanced technologies has greatly reduced the risk of HIV transmission but cannot yet detect all infected donors or prevent all transmission by transfusions. Men who have had sex with men account for the largest single group of blood donors who are found HIV positive by blood donor testing. Even taking into account that 75% of HIV infected men who have sex with men already know they are HIV positive and would be unlikely to donate blood, the HIV prevalence in potential donors with history of male sex with males is 200 times higher than first time blood donors and 2000 times higher than repeat blood donors. Men who have had sex with men since 1977 have an HIV prevalence (the total number of cases of a disease that are present in a population at a specific point in time) 60 times higher than the general population, 800 times higher than first time blood donors and 8000 times higher than repeat blood donors (American Red Cross).The deferral for men who have had sex with men is based on the following considerations regarding risk of HIV: