I don't know how much it physically hurts them but if it goes untreated they can loose a lot of fur and I think they can also get sores from it too. I know that Reachoutrescue's Tracy had a lot of kittens with it and it became so bad that one of them was almost bald. She had to take them to a no kill shelter so they could be treated because she just couldn't do it alone. You may want to pm her about more info about it. Also another pet talker Lizzie still has several kittens with it too and has been treating them for quite a while. The ringworm seems more resistant this year. Good luck.