Mine were regular dual purpose breeds. I ordered them in March. I also had a batch from Ideal and some from my regular flock and they did not experience the same mortality, so it was something to do with shipping or possible vaccinations. The invoice said they were vaccinated and I called them to ask about it and they said since it was stamped on there they were "probably" vaccinated but were certain if they were and what for.
I did get my money back for the losses but I still felt bad for all of them. They were so sick.
ETA I do think it was a fluke thing, though, and would not let that deter you from ordering the Cornish from them, especially at that price (about $30 for 25 can't be beat)