One of my small rats which I bought to raise as a breeder has just passed away today. The bunch of them were sneezing for a while but I thought it may have been the new environment + me letting the cage get a little too dirty. Since then I've been cleaning regularly and it seems like on some days the sneezing is better whereas others it's worse. None of them exhibit any other symptoms, no coughing, wheezing, or discharge. Today I discovered that one of the rats looked really skinny, and her breathing wasn't clear. I picked her out and she actually passed away as I was examining her. Does this sound like a bacterial infection?

I haven't lost any other rats yet, though I did lose 2 mice the day that I bought them. I placed them into their habitat and the next day I found two of them buried by the other mice. There were no bite marks or anything and these two individuals did not look skinny or sick at all. A similar case to the rat happened with another mouse from the same enclosure. I noticed how skinny she was and thought her mates were bullying her so I took her out to feed her. After nibbling on some greens she ended up passing away within the hour as well. I don't hear the mice sneezing at all.

Does anyone have some insight on what could be happening and how to cure it? I have doxycycline and tetracycline on hand (aquarium grade) and have ordered some amoxicillin as well.