Sudden Death -- Multiple Rats

Question
About six months ago, I adopted a couple of rats from someone.  They were young adults, about one year old, and appeared very healthy.  They were alert, no outward signs of respiratory illness, etc.  About three weeks later, I found both of them dead at the same time.  I never determined the cause, but my vet guessed something went wrong with their food.  I tossed all the rat food in the house, and I got rid of the cage they were in, just to be safe.  Eventually, I got more rats.

About one month ago, I had a healthy male, about 1 year old, who had just lost his cage mate.  I adopted a pair of very young (6 weeks or so) males to introduce to him, and they got along fine.  They lived together for a couple of months, and then all three died within 24 hours of each other.  I found the older one and one of the babies dead one morning, and the other baby seemed lethargic and cold (he had his hair all poofed out).  The other baby was dead a few hours later.

Both of these incidents happened in the same apartment.  I moved into this place about 9 months ago -- right after the building was built, incidentally, so mold seems unlikely.  I've kept rats for years, and never seen anything like this.  With the sudden related deaths, I'm assuming it is something in their environment, but I don't even know where to start looking for the problem.  The apartment is kept at a constant 70 degrees.  I feed them a diet of rat blocks, with occasional small treats from whatever I'm eating for dinner, usually veggies of some sort.  Bedding is a generic care-fresh type paper bedding.  I use open air wire cages.  I've ruled out bug spray -- I called they landlord, and they say they don't spray inside the building.  Water is from the tap, in plastic water bottles.  The rats are carefully supervised when out of the cage, and nothing about their care differed from the ways I've always cared for rats -- and I've never had problems like this before.

I'd like to get more rats, but I'm afraid to take any home until I figure out what is going on in my apartment.  I haven't experienced any unexplained illness, and my ferret has been fine the whole time (he's caged, and kept away from the rats), but clearly something is going on that is fatal for rats.

Can you suggest anything I might try or look into?  I've exhausted every possibility I can think of.  

Answer
Hi Barry

I am really sorry about the loss of your ratties. Thats just really sad.

Did you know that rodents, including squirrels and chipmunks carry dangerous viruses such as SENDAI and SDA and it is found in their urine?  All it takes is for our shoes to step in that tainted urine and we are bringing in something to our rats right on the soles of our feet.

The problem I see here is that your rats are not showing any signs of illness prior to their passing, correct?  You dont see any respiratory issue at all, such as congestion etc?

My next question is, do you notice any spiders?

The only sure way to know what happened is of course to have a necropsy done on the deceased rats.   Sometimes a gross necropsy is all that is needed to be able to identify what caused the rats death.