Early scientists were not at all shy about experimenting on themselves. Isaac Newton stuck a needle into his own eye socket. Robert Hooke shoved his arm into a vacuum chamber. And in the first episode of Sacrificed from Fusion, we learn what 18th century physician John Hunter thought was an effective way to investigate gonorrhoea and syphilis.
In short, he infected himself with pus from one of his gonorrhoea patients. At least, he may have infected himself. Historians argue about that detail, and it could just be a legend. Because as it turns out, he was not averse to experimenting on his patients, which may have made him Robert Lewis Stevenson's inspiration for Dr. Jekyll and Mr. Hyde.
What's not at issue is Hunter's hypothesis. He believed that gonorrhoea and syphilis were really different symptoms of the same disease, and due to his towering reputation (and the fact that his experiment wound up giving both gonorrhoea and syphilis to his subject -- whoever it was), it was decades before medicine straightened out the confusion.