If science cannot understand what causes a simple cold then why?

Do doctors and medical personnel that lurk in the shadows of this section think they know all there is about alternative medicine. I mean they can't even cure the common cold and never tell you how you can improve your immune system, so that your body will be strong enough to fight off these diseases. Can it be that if they were to give you this info they would lose repeat customers and a whole lot of money? Eat right, Excercise and take needed supplements that have been depleted in our food by over farming and you will be strong as a horse! What's your thoughts on this subject?