There are several things that can cause cardiac problems in young people. These can range from very serious conditions such as long QT syndrome or hypertrophic cardiomyopathy or minor conditions such as mitral valve prolapse. By that I mean a floppy mitral valve or a small atrial septal defect, by that I mean a very small hole in the heart.
Some conditions need a lot of treatment or input medically, some conditions don't need any, just caution in terms of certain lifestyle issues. But if we took into account all the conditions that could potentially cause either fatality or even morbidity in the future, then I would say about one in 200 would be identified with something. Some of these could be mild things such as just congenital abnormalities of the valves.
