Qing Li, an immunologist at Nippon Medical School in Tokyo, took 12 men on a 3-day forest trip.

Then he tested their blood and found a surge in natural killer cells — the immune cells your body uses to fight cancer and virus-infected cells.

Remarkably, the effect didn't fade when they went home, it was still measurable 30 days later.

Li estimates one forest walk a month may be enough to keep those cells elevated all year.

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