From a CNN article describing how a new AIDS drug is being tested on monkeys...
http://www.cnn.com/2006/HEALTH/conditions/03/28/aids.new.hope.ap/index.html
"Specifically, six macaques were given the drugs and then challenged with a deadly combination of monkey and human AIDS viruses, administered in rectal doses to imitate how the germ spreads in gay men.... Despite 14 weekly blasts of the virus, none of the monkeys became infected."