Tuskegee syphilis experiment
After penicillin was discovered as a cure in the 1940s, researchers did not give penicillin to any study participants.  They also did not tell any of the participants about penicillin.  Many patients were lied to and given placebo treatments so that researchers could keep studying how syphilis affected the men.  The researchers did this even though they knew that without treatment, the syphilis would eventually kill the men.