Incidence of reported cold and influenza symptoms according to season. Subjects () in placebo group (light shading) reported cold and flu symptoms year-round, with most symptoms in winter. While on 800 IU/d (intermediate shading), 104 test subjects were as likely to get sick in summer as in winter. Only 1 of 104 test subjects had cold and/or influenza symptoms during the final year of the trial, when they took 2,000 IU/d of vitamin D (dark shading; from Cannell et al. [34, 35], modified from Aloia and Li-Ng [50] with permission).