Early exposure to dogs — and to a lesser extent, cats — in infancy has been shown to reduce the risk of childhood allergies. That's because their microbes — often referred to as "dog dust," or the dried flakes of skin that fall from their coat — are introduced into the home of pet owners and then influence the bacteria in the gut and "change the immune response in the airways," Susan Lynch, an associate professor of medicine at the University of California in San Francisco, wrote in her study on this surprisingly protective quality.