The Food and Drug Administration said its decision to reverse the policy was based on an examination of the latest science which shows that an indefinite ban is not necessary to prevent transmission of HIV, the virus that causes AIDS. “Ultimately, the 12-month deferral window is supported by the best available scientific evidence, at this point in time, relevant to the U.S. population,” Dr. Peter Marks, deputy director of the FDA’s biologics division, said in a statement.