Sex and whoring will do. (A sentence I didn't think I was going to write, btw.) I believe there are military forces in Africa that are 90 % HIV positive.
It won't wipe out people in the old world, but I'm seeing a long suppression period without modern medicine.
Interesting, it will surely prevent surpopulation in medieval Europe and maybe wipe out great minds. Cities will be way less populated and the population more spread. The already short life spawn will be more dimnish so people will make more child to compensate. But maybe, after some generation, more people will be immune (some people already are OTL) ! Man, so much butterfly !