Hugh,
Transmission, Prevention, and Treatment of the Flu Virus was an excellent article by JJE. Research has suggested that a Vitamin D deficiency *may* make people more susceptible to respiratory illness. “…Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D)…” With regard to Vitamin D supplementation: “…it may be prudent to maintain wintertime 25(OH)D at concentrations achieved in nature by summertime sun exposure (50 ng/ml)…the dosage will depend upon age, latitude, season, skin type, body weight, sun exposure, and pre-existing 25(OH)D levels… Administration of 4000 IU/day of vitamin D for more than 6 months…resulted in average 25(OH)D levels of 44 ng/ml and produced no side-effects other than an improved mood…” but that “…supplementation with 5000 IU/day during winter but less, or none, during the summer to obtain 25(OH)D levels of 50 ng/ml…” may be required. The research also demonstrates that “…These studies indicate short-term administration of pharmacological doses of vitamin D is safe…” and that “…pharmacological doses (1000-2000 IU/kg per day for several days) may be useful in the treatment of those viral respiratory infections that peak in wintertime…” For a 175 pound person (~79 Kg), that dose would be 79,000 to 159,000 IU/day! If you obtain health care at Kaiser Permanente, they will test for a Vitamin D deficiency upon request. I was deficient and have been taking 4000 IU/day (I avoid the sun even in the summer) for years and no longer get the winter crud. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870528/