Chick trouble!

Hi guyz. Wonder if anyone has any suggestions re. one of my much beloved chickens. We've had her a few months now, but she has had the runs and quite badly, even tho' she lays regularly. We have several other hens and all are okay; their diet is wonderful - grains and fruit and veg. We regularly bathe her (Harriet, that is ;-)) and clean her up as it makes her vent quite sore, but she's in great health other than this. Is there anything we can give her or use on her vent to help her and hopefully clear this up? Please, please any help would really be appreciated.