Upper respiratory infections can come from any number of things, it's hard to tell what. Rats with URIs need to be put on antibiotics for a full three weeks. If left untreated, it quickly turns into pneumonia and the rat can die from it. Like Stephanie said, if you hear any congestion or excess sneezing from your rats, take them to the vet ASAP.
You should educate yourself on airborne viruses such as SDA
- both are killers if not caught early enough and aggressively treated with antibiotics and other medications.