How Do You Know When A Cat Is Ill?

Usually when a cat is ill it will have an air of disinterest about it and often be off its food, or will only be interested in drinking water.
It may lose interest in grooming itself and just want to sit away from its usual 'spots' and generally be a little more subdued than usual.
It may not be urinating or passing stools as it normally would, so it is important to keep a check on the litter tray and if there are only small spots of urine (or the tray is awash with urine) then it is likely that the cat isn't well.
Cats are often sick and pass hair or furballs this way, so don't worry too much if your cat vomits occasionally, but if it vomits frequently, then you need to take it to the vet.
Finally if your cat suddenly gains weight or loses weight then this is also an indication of an underlying sickness.
If you do suspect your cat is ill, then always seek out veterinary assistance.