In the 1920s through the middle of the century, deliberate innoculation with the malaria parasite was used as a treatment for late stage syphilus infections. The characteristic high and prolonged fevers caused by malaria are thought to inhibit and kill the causative bacteria of syphilus. The risk of death via malaria was thought to be less than the threat posed by advanced syphilus, especially with the availability of quinine, a potent anti-malarial medicine.
From the Wikipedia article: "This discovery was championed by Julius Wagner-Jauregg, who won the 1927 Nobel Prize for Medicine for his discovery of the therapeutic value of malaria inoculation in the treatment of neurosyphilis. These treatments were finally rendered obsolete by the discovery of penicillin, and its widespreadmanufacture after World War II allowed syphilis to be effectively and reliably cured."
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