They drift down through the canopy all day long, invisible. The pines and cedars and cypress release them to fight off insects and rot, and they fill the air you walk through without your ever noticing. You breathe them in. Inside your bloodstream, something begins to shift.

In a Japanese forest in 2005, an immunologist named Qing Li set out to catch that shift in the act. He took twelve men into the trees for three days. No strenuous hiking, no breathing drills, no fixed route. They walked slowly among the trees and slept nearby. Li drew their blood before, during, and after.

Their natural killer cells surged. These are the immune cells the body uses to destroy tumors and virus-infected tissue, and after three days in the forest the men had far more of them, working far harder. A control group that walked the same distance through a city showed nothing. The trees were doing the work.