I clean the rat cages every few days because I keep them in my bedroom. They are in these cages (largest size):

http://www.bigappleherp.com/Big-Apple-Rodent-Cages

They are on Exquisicat pine pelleted litter. They have "Gnawsome Huts" (wooden hides with wooden pegs in fun shapes). They get fresh water every other day or so. They eat Native Earth 18% Protein rodent block. Once in a while they get pieces of whole grain bread or table scraps. They never leave the cages except when I am cleaning them.

The rat that died was female, younger than 6 months, and apparently healthy. I couldn't find any obvious injuries, infections, or other concerning things. I had seen her just a few days ago and she wasn't acting lethargic or listless. She's never been bred. Her parents were a russian blue female to a siamese male. Her cagemate, an american blue hooded female, is doing fine. Her brother (in a separate cage) is also fine.

WTF?