Being in and around nature helps your body's immune system and boosts disease-fighting cells to act faster.
Li, Q. (2010). Effect of forest bathing trips on human immune function. Environmental Health and Preventative Medicine. 15(1): 9-17.
Li, Q., Kawada, T. (undated but probably 2010). Healthy forest parks make healthy people: Forest environments enhance human immune function. Department of Hygiene and Public Health, Nippon Medical School, Tokyo, Japan.