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Mostly these are not very serious. Cats and dogs (especially dogs) can vomit very easily and also can very easily get diarrhoea. This stems from their need in the wild to immediately eat any food found before the pack moves on or before a rival steals it.
Nature has provided them with a safety mechanism to prevent poisoning: if they are suspicious about the food or it doesn't feel right they can make themselves sick by eating grass. If the food irritates the stomach it will cause the stomach to contract and therefore cause vomitting. If the food has allready passed the stomach and causes irritation of the guts, the guts will react by passing the food through very quickly and by diluting it with fluid causing acute diarrhoea.
Mostly the animal will cure itself by not eating for a day or two giving the stomach and guts time to recover.

Dogs and cats are fairly resistant to real food poisoning but they can get it in the same way that humans do so hygiene is important for them too. It is always wise to remember that some causes of food poisoning (for example salmonellosis) can be caught by us and passed on to our pets or vice versa.

A particularly nasty cause of diarrhoea and vomitting is infection with Parvo virus.