It’s time for the all-out war on bacteria to stop. Not all microbes cause diseases like smallpox or tuberculosis. In fact, exposure to bacteria early in our lives is an essential step in developing a healthy immune system, according to researchers This concept—known as the hygiene hypothesis—is not new. A recent study reported in the journal Science, though, showed just what happens when you take cleanliness to the extreme. Researchers examined two groups of mice, the first exposed to the millions of bacteria and other tiny organisms that normally inhabit our world, and the other raised in a germ-free space, like a sterile “bubble” for rodents. Mice raised in the germ-free environment had weaker immune systems, along with increased inflammation in the lungs and colon, something that occurs in people with asthma or irritable bowel syndrome. Being born in a sterile environment didn’t doom the mice to weakened immune systems forever, though. Exposing them to microbes typically seen in mice enabled their immune systems to ramp up to full strength. This only happened if they were inoculated during the first few weeks of their life. Older germ-free mice didn’t see the same benefits. The researchers aren’t suggesting that you should give up on routine hygiene—such as hand washing and vaccinations. These steps are still important in protecting you from some of the more dangerous microbes. Going overboard with hand sanitizers and cleaning products, however, may not be doing you any favors. In fact, running or biking in the mud might be just what the doctor ordered to keep your immune system primed and ready for its next fight.