They can sicken a dog as well. Onions contain substances that destroy red blood cells. The condition, known as hemolytic anemia, is caused by molecules in onions that create high levels of oxidative stress in animals' blood cells. Glutathione is quickly depleted, and the cells fall apart. Don't feed your animal baby food without checking the label first.
Some brands contain onion powder.
Chocolate has similar effects, so steer clear of chocolate and onions. The first studies on cats and dogs taking a carotene other than beta-carotene were just published. In both cases, lutein increases the number of T-cells and strengthens the immune response. Lycopene is a very common carotenoid in the American diet. It's the carotenoid that gives tomatoes their red color.