Several years ago now, I used to listen to computer security podcasts (Security Now) by a guy named Steve Gibson. He's a smart guy who's been around for a while. In fact, he coined the term "malware", and he developed software named "Spinrite" which became the industry standard for hard drive maintenance and recovery.

Occasionally, he would go off on tangents, and one of these was a personal study of "Vitamin" D3 (cholecalcipherol). He really evangelized it. I was already taking it as a supplement at the time along with E, but his enthusiasm made me up the daily dosage. Coincidence or not, I have rarely had colds, and only experienced what may have been a touch of the flu years ago. Coincidentally the one and only season that I got a flu shot.

So, D3 is in me, in its active form it is the most powerful steroid hormone in our bodies

Steve Gibson - Security Now - Vitamin D3 link to podcast page

The D3 Podcast... runs 63 minutes