Multiple sclerosis
The people who get MS are usually between 20 and 40 years old, although it can happen to older or, very rarely, younger people too. MS is significantly more common in areas of the world that are far away from the equator.  Areas far from the equator get less sunlight than areas near the equator, and the human body requires sunlight to make vitamin D for itself. This observation supports the idea that MS is caused partly by too little vitamin D. People who move from one part of the world to another when they are children are more likely to develop MS than people who do not move long distances until later in life or who never move long distances.