Onions are a kitchen staple. To many cooks, they are as essential as salt and pepper. But does this common vegetable have a dark side? Do leftover onions act as a sponge for illness and bacteria? The Internet seems to think so. Email chain messages and social media memes have been warning for years that … More Are Leftover Onions Poisonous? Viral Warning Calls It A Sponge for Bacteria.