What can cause a rat to develop a respiratory infection or respiratory illness?

How fast can it spread?

Will it spread to other pets in the house (cats, small mammals?)

Can it be transmitted to humans?

The reason I ask is because not very long ago, one of our rats had a fractured leg. He had no other symptoms and then two days after seeing the vet for his fractured leg, he dies. Tonight I go in to feed the rats and I hold one of Gazelle's babies. Then I hear one of the feeder rejects, to be breeder, females, is sitting in her cage, gasping and making a clicking sound when she tries to breath. I take her out and normally she would have tried to bite me but she was completely oblivious to the fact that I'm holding her. Her eyes are wide open and she's just gasping. Her heartbeat is really fast and her breating extremely labored. We take her out of the room and set her up in a small critter carrier we use to take animals to the vet.

We were just talking about the next plan of action and I tell my fiance I'm going in to feed the rest of the rodents and check to see if everyone else is ok. I look in Gazelle's cage and one of the babies is not moving. I'm not sure if its the one I held or not but his body is going cold. I know it wasn't like that when I had gone in before and picked up one of the babies because all were squirming.

When I checked all the rats and fed them yesterday, everyone was fine. Now suddenly I have a dead baby and one adult that is struggling to live. We cannot get to a vet tonight but we will be waking up at the crack of dawn to call the vet and schedule an emergency visit if the rat is still alive by tomorrow.