What caused the sudden, unexpected death of my 8-week old albino rat?

Question
I bought 2 rats. Baby and Spazz. A week later Baby is less than half the size she was when I got her, so I was forced to return her. The very same day, I bought another rat(Bitsy) to keep Spazz company in case Baby doesn't live. 6 days later(yesterday) Spazz died without warning.
Can you tell me why this happened, and possibly give me a suggestion as how to prevent Bitsy from getting sick, or dying also?

Answer
What caused it? I have no idea.

You bought them from a Pet Store. That was your first problem. Pet stores are notorious for breeding unhealthy, unstable animals. Who KNOWS what could have been there. It may have been a birth defect of the heart or immune system, it could have been an upper respiratory disease, it may have been a virus or bug. Other than looking for obvious signs of illness on Bitsy (wheezing, coughing, sneezing, discharge from the nose/eyes/mouth, puffy coat, poor skin quality, blue extremities (including tail, feet, nose and tongue), there is nothing else to safeguard against whatever your other rat "may" have had.

In the future, if you want healthy, well-adjusted babies that you'll have a better, healthy start with; I'd look up a local hobby breeder and buy from them rather than funding the pet stores.