I have had 8 rabbits die in a weeks time from an unknown illness. The daytime temperatures went from being in the 50's to two days of 105 degree heat, then back into the 50's and I think that the wild temperature swing stressed them out and they died from it. Not a mark on them, no runny noses or diarrhea so stress is the most likely culprit. Will have to breed again to repopulate the cages.