My go to for this is question is the `Tuskegee Study of Untreated Syphilis in the Negro Male` (aka. Tuskegee Experiments). Where government clinicians withheld the diagnose and treatment of syphilis from the 'participants'. The men were instead told they had 'bad blood'. This lasted into the 1970s. Only ending due to an upstanding epidemiologist, Peter Buxtun, going to the press.