Fleas don't generally munch on humans - they prefer small furry creatures. But if the rats were abundant and dying, the fleas would have been desperate, and delivery boy for lunch is better than starving.

Seeing as they didn't have grocery sacks back then, the fellow could have delivered them in a wagon or something, and the rats themselves could have stowed away.

I'm not sure about the sexual transmissability of exploding groinal buboes. Seeing a sizable supporating lesion would have a tendency to turn one off. Of course, people might have been less squeamish about such things back then.