Had a serious flea infestation in our flat about 18 months ago. Our cat had fleas and I treated her with crap over the counter stuff which sort of worked. Then we went away for a fortnight with the cat and the fleas, now minus a "host", all came out of the woodwork, literally. When I returned to check post etc they jumped all over me, literally - they were in my socks, in my fucking bra, those bastards can JUMP - and we had to call an exterminator; he said he'd only seen infestations this bad in abandoned buildings. Got the cat to the vet for sone super anti-flea shot, exterminator sprayed every surface in the flat, and we were told not to let the cat back, or hoover, for about six weeks. And even then there were still some clinging on for dear life.
No idea where they came from as she's a house cat and we had her for almost a year before this fiasco, but apparently fleas can be brought in on your clothes/shoes etc and lie dormant for months.
I feel your pain. It was fucking horrible.