You may have recently found out that you are deficient or know someone who is. It's shocking for most people when they have never had a problem before and believe nothing has changed to make it a problem now. The truth is that a lot has changed, and vitamin D deficiency and insufficiency is now a global public-health problem affecting an estimated 1 billion people worldwide.
The most well-known consequences to not having enough vitamin D are rickets in children and osteomalacia in adults. These are far from the only problems associated with a vitamin D deficiency.