^Immune System Boosters: Everyday, get Exercise (Stress
  reliever), Get Outside (Vitamin D + fights against depression)
  and even, get this - Get Dirty. "Exposure to friendly soil
  bacteria could improve mood by boosting the immune system just
  as effectively as antidepressant drugs" and its not just for
  kids - adults benefit from exercise, dirt, outdoors. Take Zinc.
  So says Doctor Ken (not a licensed doctor, just a guy who reads
  way too much)^