Stress describes the internal changes that occur in a cat's physiology when something changes in its environment. The stress response is a very adaptive change that enables the cat to be able to show an appropriate behavioural response in changing circumstances. Once the cat has shown a behavioural response, the stress response declines. Stress responses occur in response to both 'good' and 'bad' changes in the cat's environment. Stress becomes a problem if it lasts over a prolonged period of time, as it can lead to the development of abnormal patterns of behaviour and to certain types of disease in cats.