The things you learn when reading Wikipedia...
Apparently, for a while, giving someone malaria was used to treat syphilis—doctors had observed that in some patients, a high fever enabled the patient to recover from syphilis, and malaria leads to long and high fevers.
The risk was considered acceptable because malaria was treatable with quinine, which was available, while there were no effective direct cures for syphilis at the time (especially for later stages of the disease).
So I guess that if you had syphilis, they'd give you malaria, then after you'd been in fever for "long enough", they'd give you quinine to get you off malaria. Whee. Sounds like tons of fun.