Foodborne illness is when you get sick from eating bad food. Bad food usually has germs like bacteria, viruses, and parasites that can give you an upset stomach, fever, and vomiting. To avoid getting sick, it's important to make sure the food you eat is cooked correctly and that it has not been left out too long. It is also important to avoid eating food that has been touched by someone who is sick, or that has been left in warm places where germs can easily grow.