What it is
The pancreas makes digestive enzymes and hormones, insulin among them. When it becomes inflamed, the enzymes start working too early and irritate the pancreas itself and the tissue around it. The inflammation can flare up suddenly or smoulder for a long time, sometimes together with inflammation of the gut or liver.
The cause is not always found. Cats of any age can be affected, and long-standing inflammation can damage the cells of the pancreas that make insulin.
The greatest danger for a cat is going without food: a cat that eats nothing for several days can develop serious liver disease. So even a quiet loss of appetite with no other signs is a reason to see a vet.
Signs you may notice
A cat stops eating, turns listless, hides and sits hunched. It may vomit, have diarrhoea and lose weight, although some cats do not vomit at all.
When to see a vet
If your cat has eaten nothing for a day or more, contact a vet the same day, even with no other signs. If it vomits again and again, is very listless or its gums look yellow, a vet is needed immediately.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.