Answer: Malaria

Before 1917, syphilis was a terminal disease: there was no treatment and the effect on your brain was essentially a descent into madness. All of that changed when Julius Wagner-Jauregg, an Austrian physician, tested his theories about pyrotherapy (the use of high fevers to incite immunological responses in individuals and cure them of illnesses unrelated to the cause of the fever), on syphilis patients.

Specifically, Wagner-Jauregg infected the patients with malaria in order to induce a high fever. Although a malaria infection is no walk in the park, there were cures available where as syphilis was incurable. The gamble paid off and the intense fevers that accompanied the malarial infections prompted the patients’ bodies to attack the syphilis infection too. Wagner-Jauregg received the 1927 Nobel Prize in Medicine for his discovery.