Because it works by an inherent mechanism that stifles autoimmune response as well as viral reproduction. The fact that Plasmodium (malaria) is susceptible to the medicinal properties of quinine does not negate its efficacy, even by different mechanisms, against other diseases.
Fine, if you test positive you can take any meds your doctor may prescribe, I will treat it like any virus, rest, keep warm, and let the fever break.