A very unusual blood transplant appears to have cured an American man living in Berlin of infection with the AIDS virus. The man, who is in his 40s, had a blood stem cell transplant in 2007 to treat leukemia. His donor had a gene mutation that confers natural resistance to HIV. “It’s an interesting proof-of-concept that with pretty extraordinary measures a patient could be cured of HIV,” but it is far too risky to become standard therapy even if matched donors could be found, said Dr. Michael Saag of the University of Alabama at Birmingham.