Many studies show that vitamin D plays an important role in the body's immune system. If your baby is deficient in vitamin D, it will increase the risk of infection. One of the most important functions of vitamin D is to activate T cells (which are killer cells). This cell is responsible for detecting and destroying any viral pathogens from outside.
Therefore, vitamin D also contributes to helping the body "fight" against disease-causing bacteria. This is also the reason why vitamin D deficiency will directly affect immunity. Especially respiratory infections.
The best way for parents to protect their baby's health is to strengthen their immunity. In which, adequate supplementation of vitamin D is really necessary.