Summarize the following:
However, rats are known to have very weak respiratory systems and it's not unusual for your rat to get sick at some point in their life. If you hear your rat sneezing, coughing, wheezing or constantly making a deep purring sound, or if you see discharge around the eyes or nose, take your rat to the vet immediately. Your rat will need antibiotics and/or antivirals to prevent other infectious bacteria and viruses from taking advantage of an already compromised immune system. Always keep an eye out for signs of illness in your rat but especially following times of high stress such as a move, separation or death.

Summary:
Keeping your rat's cage clean, refraining from using dusty litters and woodchips, and removing uneaten food both go a very long way in keeping you rat healthy.