There haven't been any reports stating that people have become sick from jello, but there are some safety concerns that you should be aware of. If jello is not refrigerated, bacteria can start to grow and multiply. This is especially true if the jello is made with milk or cream, and if you are unsure whether it has been refrigerated, it is best to throw it out. While there is no need to refrigerate jello, doing so will help to keep it fresh and prevent bacteria from growing. It is best to refrigerate if you are serving jello to young children or those with weakened immune systems.