According to this review by Romanian and British research institutes, vitamin D can benefit immune response (the reaction which occurs within our organism to defend against foreign invaders) in respiratory viral infectious diseases. This is because patients with respiratory infections are often vitamin D deficient. As a result of its increased antiviral and anti-inflammatory responses, vitamin D can prevent acute respiratory infections and common viral respiratory infections in both healthy people and patients with chronic respiratory diseases. Finally, vitamin D can enhance the efficacy of treatments used to prevent asthma attacks and other symptoms of chronic lung diseases. More details can be found in the review.
Zdrenghea et al.