A 100,000 years ago? I'm a little confused. Although we started migrating 60,000 years ago from Africa it wasn't until about 45,000 years ago that our species colonized Europe where Neandertals lived (There is no evidence of them elsewhere). There may have been other "archaic" species in Africa who, when we began to expand in and out of the continent, might have infected us.
Yet again, we might have caught it from Neandertals in the ever changing border between our two populations (?).