Undeniably a factor, but
***the drugs must exert some activity/mechanism that also fights this disease.***
as you pointed out, we seem to be fitting a square peg into a round hole. Further study might yield highly effective drugs exclusively against both parasites and viruses. As I understand, it is viruses where we are woefully short in effective treatments. At least I am curious as to how an anti-parasitic - a cellular form of biological life - can defeat a non-cellular molecular form of biology. Then too, penicillin came from bread mold.
Still, perhaps you are right.
Well, in human history, ‘Impossible’ has often just required Will and Faith. There aren’t many diseases that we haven’t found ways around, or at least ways through which we have lessened effects and prolonged life.
After all, it’s not as if we were trying to do what’s been proven impossible - and sometimes, approximations can be very helpful:
https://en.wikipedia.org/wiki/Squaring_the_circle