Other forms: cross-contaminated; cross-contaminating; cross-contaminates
To cross-contaminate means to accidentally transfer germs or allergens from one thing to another. It's a bad idea to use the same, unwashed knife to slice raw meat and then an apple.
If you make a sandwich on the same cutting board that you just chopped up raw chicken, you risk cross-contaminating your sandwich with a dangerous bacteria like salmonella, which can cause food poisoning. While some kitchen mistakes, such as getting the curry flavor from your dinner party stew into your dessert cake, just result in a bad-tasting dish, cross-contamination is a serious health risk. To stay safe, always wash your hands, utensils, and cutting boards thoroughly between cooking tasks.