Vitamin D3 Supplementation to Prevent Respiratory Tract Infections

Inclusion Criteria: * Hospital worker * Age 18 years or older Exclusion Criteria: * History of hypercalcemia * History of nephrolithiasis * History of intolerance to vitamin D3 supplements * Use of calcium at a dose \> 600 mg/day (individuals using a dose greater than 600 mg of calcium per day will be asked to limit the amount to 600 mg unless they have been directed by their physician to be taking more than 600 mg/day of calcium. If the latter is true the potential subject will be excluded from the study.) * Use of vitamin D at a daily dose \> 5000 IU\* * Use of aluminum-containing phosphate binders in patients with renal failure * Use of calcipotriene * Use of digoxin * Use of thiazide diuretics if using: * hydrochlorothiazide at a daily dose \> 37.5 mg * indapamide at a daily dose \> 1.25 mg * chlorthalidone at a daily dose \> 12.5 mg * metolazone at a daily dose \> 2.5 mg * methyclothiazide at a daily dose \> 2.5 mg * chlorothiazide at a daily dose \> 250 mg * metolazone at a daily dose \> 0.5 mg * bendroflumethiazide at a daily dose \> 2.5 mg * polythiazide at a daily dose \> 1 mg * hydroflumethiazide at a daily dose \> 25 mg * Conditions that are associated with a risk of modified vitamin D metabolism * Known allergy to wool * Current enrollment in another study * Life expectancy \<1 month at time of screening * Cognitive impairment precluding the ability to provide informed consent * Pregnant or trying to become pregnant * Employee is team member on the present study * If potential participants are found to be using vitamin D supplementation upon screening at a daily dose ≤5000 IU/day, they will be eligible for participation by switching to the study dose. If potential participants are taking a multiple vitamin or calcium supplement and there is less than or equal to 800 IU vitamin D in it, they can continue the multivitamin or calcium supplement along with taking the study vitamin D3. Total vitamin D cannot exceed 5,800 IU per day combined between any supplements that contain vitamin D. Use of vitamin D at a daily dose \> 5000 IU at the direction of a physician will be an exclusion criterion. If a potential subject uses over-the-counter vitamin D not directed by a physician at a daily dose \> 5000 IU they will be eligible to participate by switching to the lower study dose.