4. You find that you are getting sick more often
Of all the functions vitamin D plays, strengthening the immune system is one of the most important. A strong immune system is necessary to ward off illnesses and infections. If you are deficient in vitamin D, your immune system will become weak. As a result, you will be more prone to developing various illnesses and infections that are caused by bacteria and viruses.
You might find that you are developing the flu, strep throat, or colds more often than you normally do. There is also a chance that you could be more prone to developing a chronic illness, such as COPD. One study revealed that those who suffer from COPD have very low levels of vitamin D.