From 1946 to 1948, US researchers led by Dr. John Cutler (who also ran Tuskegee) deliberately infected 1,300 Guatemalans - including soldiers, sex workers, orphans, and mental patients - with syphilis, gonorrhea, and chancroid without their consent. Only 700 received any treatment. The experiments were hidden until historian Susan Reverby discovered them in 2010, prompting a formal presidential apology.