Let's face it: Toddlers are germ magnets. They're drippy and drooly and, as gross as it may seem to you, they happily put almost everything in their mouth. Fortunately, experts agree that most germs are harmless -- and may even be helpful. "Humans actually need exposure to good germs early in life to prime our immune system so it develops properly," says Elizabeth Scott, Ph.D., co-director of the Simmons Center for Hygiene and Health in Home and Community at Simmons College, in Boston. Still, to help your kid (and the rest of the family) stay healthy, it's important to keep your toddler's world relatively free from those germs that can be dangerous. Not sure what you need to scrub and what can be swept under the rug? We've got the dirt on all things dirty right here.