Sneezing chicken with discharge

[15gardenchooks]

I just noticed that one of my hens has started to sneeze repeatedly. She also seems to have some sort of discharge around her face and has a really gross vent area. All the feathers are wet or covered in poo. she seems to be acting different than normal as well, but still eating. Some of my other chickens have the same thing going on on their rear ends. I recently saw one of them lay an egg without any membrane or shell so I thought that might be it. They are all at least a year old, and the rest seem to be doing fine apart from an occasional sneeze. I do also have some chicks that have been sneezing a little, should I be worried?

I'm mostly concerned about the really sickly hen, what should I do?