From 1946-48, the US Public Health Service and the Pan American Sanitary Bureau worked with several Guatemalan government agencies on medical research paid for by the US government that involved deliberately exposing people to sexually transmitted diseases.
The researchers apparently were trying to see if penicillin, then relatively new, could prevent infections in the 1,300 people exposed to syphilis, gonorrhea or chancroid. Those infected included soldiers, prostitutes, prisoners and mental patients with syphilis.
http://www.guardian.co.uk/world/2011/aug/30/guatemala-experiments
