Consumption of North American ginseng (Panax quinquefolium) appears to confer some immunity to catching colds. Two hundred and seventy-nine subjects were randomised to receive either 400mg of freeze-dried ginseng or placebo for four months. Consumption of the ginseng was a associated with a decreased mean number of colds and reduced total symptom scores and days of illness. (Predy GN et al. Efficacy of an extract of North American ginseng containing poly-furanosyl-pyranosyl-saccharides for preventing upper respiratory tract infections: a randomized controlled trial. Can Med Assoc J 2005; 173: 1043–8).