According to the so-called hygiene hypothesis, identified in the late 1980s, growing up indoors in disinfected spaces later causes our bodies to over- react to harmless substances, making us more prone to allergies and asthma. Dogs and cats, with their muddy paws, copious dander and propensity for licking us, introduce more microbial diversity that habituates our immune systems. Research shows that children who live with pets from birth have lower rates of allergies and asthma, and the more animals in the house, the greater the protection. Kids with four or more cats or dogs had half the rate of allergies as non- pet owners.

Even in adulthood, there’s new evidence to suggest pets may have a notably positive effect on our guts—with links to both mental and physical health. One pair of researchers at the University of Arizona are studying whether the sharing of bacteria between dogs and their owners can alter our microbiome—the community of micro- organisms that dwell inside our bodies—in a way that changes our brain chemistry, alleviating major depression. “We were intrigued by previous research that found that dogs and humans share gut bacteria just by living in the same home, and you get the same amount from your dog as you do from your spouse,” says Dieter Steklis, co-director of the Human-Animal Interaction Research Initiative.