Injured cat

Got a call last night from one of the local charities to contact someone who has a stray who appears to be injured, with raw skin and missing fur near it's tail. The person who feeds that cat has preciously borrowed a trap and cat and neutered this cat so he is wary of going into a trap again. He went near a trap the RSPCA brought the other day, but it went off when he was still outside so scared him away. They were almost wanting me to 'diagnose' the cat over the fone as they are worried it may have something contagious... eh. not a vet, sorry. Have told them I can bring a trap round once the cat returns and we can put it on manual release so may have more chance of catching it... we'll see.