The Tuskegee syphilis study was an unethical medical experiment conducted between 1932 and 1972 by the U.S. Public Health Service to study the natural progression of untreated syphilis in African American men who thought they were receiving free health care from the U.S. government. Over 600 men infected with syphilis were studied without their informed consent and were intentionally denied treatment even after penicillin became the drug of choice for syphilis in 1947. The study violated basic medical ethics and caused undue suffering, making it one of the most infamous cases of unethical human subject research in U.S. history.