What has been the most horrible experiment that has been done on human beings?

 There is a horrible medical experiment that a doctor performed… on himself.

John Hunter was a doctor who studied sexually transmitted diseases.

The famous English surgeon John Hunter (1728-1794) was a doctor who studied sexually transmitted diseases. At a certain point he determined to follow the clinical course of gonorrhea. He hypothesized that gonorrhea and syphilis were one and the same disease. In 1767, using pus from a student's urethra, he inoculated his own. He had the added misfortune of choosing a patient who suffered not only from gonorrhea but also from syphilis and the hard chancre was probably located at the opening of the urethra. For the next nineteen years Hunter followed the development of his illness. He published the results of monitoring him in 1786. He was sure that gonorrhea and syphilis were the same disease, which caused his death in 1794."

The poor man wanted to prove that syphilis and gonorrhea were the same disease; and he had the bad luck to choose a patient who had both diseases. He experimented for nineteen years (I know the dates don't add up, probably the last few years he was so sick that he couldn't help but curse his investigative emphasis at the expense of his health) on himself the crossed symptoms of the two, published his results profusely. and, given his enormous prestige, for seventy years hardly anyone questioned his claims.

In short, he did a disservice to venereal disease research—and to himself.