In general, vitamin D functions to activate the innate and dampen the adaptive immune systems with antibacterial, antiviral and anti-inflammatory effects. Low levels of vitamin D appear to be a risk factor for tuberculosis, and historically it was used as a treatment.
Vitamin D supplementation in low doses (400 to 1000 IU/day) may slightly decrease the overall risk of acute respiratory tract infections. The benefits were found in young children and adolescents (ages 1 up to 16 years) and were not confirmed with higher doses (>1000 IU per day or more). Vitamin D supplementation substantially reduces the rate of moderate or severe exacerbations of COPD in people with baseline 25(OH)D levels under 25nmol/L, but not in those with less severe deficiency.