As the Diagram shows, Vitamin-D has many effects that could benefit each and every person. Dr. Purcell recommends that each person should take at least 5,000IU of Vitamin-D daily; 10,000IU during the winter months. Regularly taking Vitamin-D can assist the body in fighting illness as well as prevent the initial onset of symptoms.
The diagram is from the Mayo Clinic Proceeding; article from March 2006; 81(3): 353-373. Attached in the link to read the full paper explaining all the benefits of Vitamin-D.