On the other hand, the behaviors cats manifest as a result of stress are extremely varied, and depend on the individual cat. Blatant signs like uncharacteristic aggression of shyness, or more subtle symptoms like tail chasing, hiding or any type of repetitive, nonproductive behavior could be stress related. If a cat is under stress and isn't helped, it could develop a stress-related illness. When a cat becomes stressed and the level of corticosteriods, cortisol in particular, remains high for a prolonged period of time the cat's immunity decreases making it more susceptible to serious diseases. Some of these diseases are predestined, meaning they are already in the cat's system, but are latent, or hidden.
Latent diseases tend to come to life during times of stress.
Dr. Reisner says some cats carry a latent upper respiratory virus. When the cat is boarded and comes home sneezing with an upper respiratory infection, people assume that the cat picked up the infection at the kennel. This isn't always true. The stress of being boarded may have triggered the latent virus. Stress causes other health problems, too.