Full article in link below:

https://www.bbc.com/future/article/20220929-how-outdoor-play-boosts-kids-immune-systems

Dirt is teaming with friendly microorganisms that can train the immune system and build resilience to a range of illnesses, research shows (Credit: Getty Images)
By Alessia Franco and David Robson11th October 2022
Children love getting dirty. They are drawn to puddles like muddy magnets, with no regard for footwear or the colour of their clothing. But getting mucky could have a powerful effect on their wellbeing, too.