Sorry to hear about your flock.
I know you won't like what I am about to say, but that is okay. We are all entitled to our opinions. But seeing as the entire flock is sick and in light of the recent death, I would cull everyone, sanitize and begin over again with healthy birds. At the end of the day, I simply cannot justify vet visits for a $10 chicken. The more logical and economic option for me is not to harbor disease in my flock, and to simply rid myself of the problem. Otherwise, i would have to potenially wait for every last bird to die on their own so I can be assured of a healthy flock before I ever get more birds. I don't have the time or money to waste doing that.