> 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.

In that light, does it make sense to speak of "boosting the immune system"? If I understand correctly, allergies and asthma are the result of overly active immune system. Depression and anxiety are completely unrelated to the immune system as far as I know. So if anything, playing in mud dampens the immune system of children, albeit in a good way.

Also, the title of the article neglects to mention that this only applies to children.