What should my cat not eat? Foods poisonous to cats

Which foods are better to avoid to give your cat or might even be poisonous? Below you will find some examples of foods poisonous or harmful to cats.
Onions
Onions contain the substance allyl propyl disulfide, which destroys cats’ red blood cells. This causes a form of anaemia. Garlic also contains this substance, but to a lesser extent.
Chocolate
Theobromine is a substance in chocolate that is extremely poisonous to cats. It can cause diarrhoea, vomiting, anxiety and (in extreme cases) fits.
Grapes & raisins
Although fruit is not generally bad for cats, it is better not to give them any grapes (nor raisins). They contain a substance that is bad for cats. It is not yet known what substance is the culprit, but prevention is better than cure.
Milk
Most cats are lactose-intolerant. What this means is that lactose (milk sugar) and dairy products can cause flatulence to your cat.
Raw eggs
Please do not give your cat raw eggs. Eggs contain avidin, an anti-vitamin that adversely affects the metabolism of fats, sugar, amino acids and energy.
Seasoned / marinated meat or fish
Avoid giving your cat meat or fish that is in any way salted, seasoned or marinated. Seasoning often contains onion or garlic, which is bad for the red blood cells of your cat, and can cause anaemia.
Plants
Some plants may be poisonous to your cat, such as lily, yew, tulips, holly, apricots, azalea and arum. To see the full list of plants that are toxic and why, check this page.