Prince Erik of Sweden was also an epileptic like Prince John, and I think I read somewhere that he also had severe learning disabilities, or may have been mental retarded, because his mother took medications during her pregnancy with him that she probably shouldn't have, but doctors probably didn't know would have any adverse effects.
Princess Charlotte of Prussia and her daughter, Feodora, both I think were confirmed to have porphyria, which is what made George III so,... crazy.
There were also a lot of hemophiliac princes. Alfonso, Gonzalo and Alexei , there was also Prince Waldemar and Henry of Prussia (the nephews of Kaiser Wilhelm, not his brothers) Prince Leopold of GB, Prince Rupert of Teck, Prince Friedrich of Hesse, and Prince Leopold of Battenberg. I think I may have read that Princess Sibylla of Luxembourg's brother Paul, who died young, might have been a hemophiliac, but I don't know.
http://www.abc.es/informacion/boda/r...pana/index.asp
and read the profile of Queen Victoria Eugenia
I don't know if this counts but Alfonso Pio (*1907,1938) and Gonzalo (*1914,1934); the eldest and the younger sons of King Alfonso XIII of Spain were hemophiliacs.
Zarevich Alexei, the heir of Nicholas II of Russia was an hemophiliac as well
Prince John suffered from epilepsy and, most likely, Asperger syndrome which is a form of autism.
He was 13 years of age when he died in 1919.
Prince John Charles Francis was the youngest child of Their Majesties, the King George V & Queen Mary (formerly, Duke & Duchess of York).
The little Prince resided at Wood Farm on the Sandringham estate and was well taken care of until his passing, especially by his devoted nurse & confidante, Miss Charlotte Bill. (there is a movie on him which you mentioned--The Lost Prince-us that the one?--very touching movie)
Most give up the throne not being able to fulfill all it demands, often care for by away from public eye
Many far the past. (who were known to to crazy--may have had more of a medical condition that we know more about today) Many just seem to have accidents or killed.
(Hate to point this out but there was a lot of close breeding in the royals in times past. That's not good for man or beast.)