Sepsis is a clinical entity that complicates infection. Without early recognition and timely management, it can rapidly progress to severe sepsis, septic shock, and culminate in multiple organ dysfunction syndrome. Forty to 70% of septic patients have low vitamin D status, yet little is known about the impact of vitamin D3 (vitD3) supplementation in this patient population. As such, the investigators propose a randomized, double-blinded, placebo-controlled trial to test the hypothesis that early, rapid correction of low vitamin D status, as an adjunct to established treatment guidelines, will improve clinical outcomes and measurably alter immune profile in patients with severe sepsis.