Sudden changes in behavior, bowel movements, appearance and appetite are prime indicators of illness in cats, according to Petfinder. Bad breath can also indicate the development of dental disease or kidney problems, while sweet and fruity breath is a symptom of diabetes.
WebMD warns pet owners that cats are more likely to hide their symptoms, especially if they are aloof or independent already. Cats should be watched carefully; cats often vomit for benign reasons, but chronic and reoccurring vomiting indicates an infection or even an obstruction in the gastrointestinal tract. Cats can eat bones, string and other small objects. Bloody stool or diarrhea is a warning sign that a cat needs to see a vet immediately.
Petfinder recommends observing cats to see if they have any difficulty or pain associated with using the litter box. Lower urinary tract disease can make urination painful, and some cats associate the pain with the litter box, causing them to seek other areas of the house to relieve themselves. A decreased appetite can be very dangerous in cats, who can only survive for a few days without food before putting themselves at severe risk for fatty liver, leading to liver disease and death. If a cat goes without food for 24 hours, one should seek medical attention.