Fleas. You are very lucky.

I once moved into a flat which had fleas. Each night I walked round all the rooms with bare legs and a glass of soapy water. As each flea leapt on to my legs, I picked it off and dropped it in the water. First night there were about 70, fewer on each succeeding night. It only took about a week. I can't remember if I supplemented it by flea powder in likely places. It probably would have been the sensible thing to do.