Source
https://www.sciencealert.com/daycares-in-finland-built-a-forest-and-it-changed-kids-immune-systems
When daycare workers rolled out a lawn, planted forest undergrowth (such as dwarf heather and blueberries), and allowed children to care for crops in planter boxes, the diversity of microbes in the guts and on the skin of the young kids appeared healthier in a very short space of time.
Compared to other city kids who play in standard urban daycares with yards of pavement, tile, and gravel, 3-, 4-, and 5-year-olds at these greened-up daycare centers in Finland showed increased T-cells and other important immune markers in their blood within 28 days.