Not only humans suffer from stress. Animals too. Under adverse conditions, like trips, change of environment or routine, animals can react with physiological and behavioral changes.
Some animals present diarrheas when they come back from pet shops beauty session. For those animals, the situation generates stress that can cause them diarrhea, even after the best treatment ever offered. In those cases, it is advisable the accompaniment to the animal by his master, during the bath.
The arrival of a new animal at home can be a stressful factor too, no matter the kind of animal. There can be a behavior change, with signs that vary from aggressively to apathy. Physiological changes may occur with vomits, diarrhea or loss of appetite.

The master´s death is a situation of extreme stress. Many animals refuse to eat for several days and they lose the interest on everything surrounding them. There are cases where the animal gets ill and may even die soon after the master’s death.
Situations that may cause stress in dogs and cats: the master’s absence, time or frequency reduction of the walks, change of an employee at home, house renovation, or fewer hours the owner spends with the animal.

After suppressing the cause of stress, most of the animals get back to their normal life. The stress is not an "illness" but rather a common state in animals. It may reduce the immune system resistance leading to an illness. When the pet is ill or have changes in behavior, we have to also check for other probable causes before blaming it on the stress.