give us a run down of what you are using for the entire setup. I'm talking bedding, food, cleaning agents, everything.
I breed rats as food and pets and have not had any fatalities. If you placed a mouse in that enclosure and it died within 24 hours you have a serious issue on your hands. My 2 guesses are either:
1. Something you are using in your setup is highly toxic. Be it your bedding, or the soap you used to clean everything.
2. you are dealing with one hell of an ugly viral infection. This one is a little more unlikely but still possible if your rodents are dying that fast.