I know things seem really bad now but understand it has happened to all of us to some extent. Sometime illness, varmits, and sometimes we never figure out what went wrong.
Wanted to let you know I too give bread to my chickens. I also give broccolli, cooked rice, lettuce, cucumber peelings, stale raisin bran, the list goes on and on. I am sure it was safe providing it was not moldy. Maybe with this heat.....
I have lost 60 chicks in a matter of a week because of feed that was molded at the bottom of the bag. It hurts the heart more than the pocket book. But I hope you don't give up. If you had a chicken with cocci, I do recommend you research on here the cleanup process. It extensive and timely so you may not want to jump in with new chicks just yet. I think everyone has experienced that too if they have raised chickens for any amount of time.
I am no expert but there are a lot of peeps on here that can help you with the bio-security measures on that thread. Big hugs
and hang in there!