I cant say if I'm right or wrong on the cocci thing. I'm just going by the general knowledge of the illness. cocci normally wont affect a chicken over 8 weeks of age, and typically doesn't just affect one bird in the flock. I think this problem is more toward a bacterial problem. we talked on the phone for a little while, I think we ruled out egg binding; but I'm not sure if she checked. you can tell if a chicken is holding an egg by feeling between her legs to the area under the vent. if they have an egg you can feel it pretty plain. never squeeze to hard when doing this as there could be in infection that you can shoot up into their system - and cause toxic shock syndrome.
if ohfarmerswife had the same issue, I think the corid worked for another reason. it is an antibiotic, I'm not sure of what all it will kill, but it may have killed whatever it was if it was bacterial. it also has something in it to aid b vitamins- b vitamins make all animals hungry, getting it to eat is a huge step to getting it back on its feet.
I don't know how corid affects e vitamins, but this time of year a lot of flocks suffer from e deficiency. the symptoms will be stroke like, or possibly wry neck. the reason they suffer from it - most chickens get their e vitamins from grain, when its hot they wont eat the whole grains as often. broccoli, whole wheat, and BOSS helps with this.
another thing not to overlook in this case: black chickens, and 100 degree temperatures don't mix well. for anyone who isn't doing it, I recommend putting some vitamins and electrolytes in their water. Gatorade and pedialyte work well too.
when I have this issue with my own flock, I typically give tylan for 3 days, along with b and e vitamins. I'm not sure what part of the treatment works, but it works 9 out of 10 times. if the chicken gets so bad as the neck turns (wry neck) I use injectable vitamins.
after any of these treatments, if the weight is low I typically worm with a small amount of tobacco. if your not worried about being organic you can also use ivomec pour on for cattle, it works well for lice, mites and worms. - it may temporally affect fertility. ive looked at 7 flocks this past week, 6 had mite problems. mites typically wont kill a bird, but will cause anemia; and cause problems that typically wont hurt your chickens to manifest in the worst ways.
I'm not putting this here as an argument, so please don't take it that way. I still have a lot to learn, but I have also been though a lot. keep in mind using the wrong antibiotic or to much antibiotics may cause resistance when you need them.