Biohazard.

When the mages of Arch summoned a hero, they expected a weak willed teenager. They had made homonculus that would tempt all but most chaste person. They created a fake adventurers guild filled with demons disguised as mortals. Eventually, the hero was summoned. Unfortunately they forgot to update the hero's immune system, or clear them of disease. Luckily the hero had an entire college course on disease. Watch as a hero fights an enemy formerly untouchable in Arc.