This little cat ran up to me this morning. He's got 4 sores like the one on the photo, located on his hip near his tail, one in his shoulder, and one either side of his body. All in all, the full bald patch is close to 1 inch in diameter, the sore itself is a bit smaller. The RSPCA won't do anything without knowing more, so have told my DP who will keep an eye out for it so he can catch it and take it to a vet (I can't, am off on a work trip abroad).
Thing is, I am worried it might be ringworm, and I don't want this to be transferred to our two cats. Anyone recognise what it could be?