Don't get dirty!" was once a constant family refrain, as parents despairingly watched their children spoil their best clothes. Today, many parents may secretly wish their children had the chance to pick up a bit of grime. According to recent research, the dirt outside is teaming with friendly microorganisms that can train the immune system and build resilience to a range of illnesses, including allergies, asthma and even depression and anxiety. Certain natural materials, such as soil and mud ... contain surprisingly powerful microorganisms whose positive impact on children's health we are only beginning to fully understand. Our brains evolved in natural landscapes, and our perceptual systems are particularly well suited to wild outdoor spaces. Supporting this theory, one study from 2009 found that children with attention-deficit hyperactivity disorder (ADHD) were better able to concentrate following a 20-minute walk in the park, compared to a 20-minute walk on the streets of a well-kept urban area. People who grow up on farms are generally less likely to develop asthma, allergies, or auto-immune disorders like Crohn's disease [due to] their childhood exposure to a more diverse range of organisms in the rural environment. Michele Antonelli, a doctor from Italy ... has researched the ways that mud therapies can influence health. People with [skin] disorders ... seem to have an impoverished community of organisms. "These microorganisms can play a major role in many major chronic diseases," he says.