My cat's had a similar thing happen just the other week. He's a semi-longhair, and gets the occasional knot of fur which I have to tease out, but I'd noticed that this had happened three times in the same spot - in his ruff, just under his chin - over a few days. Looking under the fur, there was a small spot of bare, slightly reddened skin about the size of a pea. Around this, the fur had been thinned out.
The vet examined him, and said that he couldn't find any other spots like this so felt it probably wasn't a skin infection. He didn't say what it was but advised me to treat it as a 'one-off', and gave me some gel to apply to the spot for a week and see how the cat got on.
The gel was only to alleviate any irritation - the hair loss was due to the cat scratching himself. It might not have been an explanation of what the cause was, but the cat seems fine now; his skin's calmed down and he hasn't lost any more fur.
The only thing that he gets attacked by is me, and neither of us have any fleas either.