AIM 2: To determine if early administration of vitD3 (vs placebo) modulates b25OHD, AMPs, and key cytokines in patients with low vitamin D status and severe sepsis. Hypothesis 2a: Early vitD3 increases b25OHD in the first 5 days after the onset of severe sepsis. Hypothesis 2b: Early vitD3 increases expression of AMPs, cathelicidin (LL-37) and β-defensin (hBD-2). Hypothesis 2c: Early vitD3 decreases pro-inflammatory cytokine (IL-1β, IL-6) and increases anti-immunoparalytic cytokine (IL-10, IFN-γ) expression.