Bless her heart that just looks awful! I'm sure she must feel better since it has been drained though. Poor baby! She is very lucky she found you or she would have been really sick outside with all that infection.
We have one at the shelter that was brought in with a huge abcess and they opened it up like that. His whole shoulder and front leg were open almost to the bone. It was really nasty to look at but he is now finally healing. It took several months but he is finally scabbed over. It never seemed to bother him once they drained it though.
Abcess's are just such nasty things that poor kitties get so easy. I'm sure glad Hazel will be ok now.