According to the People's Dispensary for Sick Animals, you can recognize sickness in rats by watching for abnormal behavior and symptoms. Symptoms of illness in rats include diarrhea, weight loss or gain, changes in appetite, sleeping more often, hunched posture, limping and staining around the mucous membranes.
The People's Dispensary for Sick Animals says that common causes for illness in rats include respiratory infections, obesity, stress and tumors. Rats can become ill as a result of poor cage hygiene or insufficient diet or they can become sick for no obvious reason. If a pet rat exhibits any abnormal symptoms or behaviors that do not resolve themselves, it is important to contact a veterinarian.