A lot of heart conditions often go completely unnoticed. My cousin in the Isle of Man had an ASD (hole in the heart), and he lived a completely normal life till the age of 15, when he started playing football and would be out of breath very quickly, and his father is a doctor. A lot of these footballers live completely normal lives until the they simply collapse one day and die.
Some of them get lucky, for example Real Madrid's Ruben de la Red fainted on the pitch but was taken to the hospital in time, and had to promptly retire. FIFA and UEFA need to be more vigilant because considering the number of players who have died due to heart problems, it seems that the regular medicals at clubs and the other health checkups are failing to detect these issues.