"The Demon Violinist" was the best known virtuoso and composer of his time. He was so talented, many decided that the only explanation was that he sold his soul to the devil.
The reason he was so good, it's now thought, is because of extreme hypermobility in his hands, which you can see in some paintings of him.
Now, it's thought he had either Ehlers-Danlos Syndrome, or Marfan Syndrome, which would explain his nickname- The Rubber Man.
Originally, his illness was referred to as "Paganini Disease", as often happens when someone famous had something a lot of people were unfamiliar with, and "Paganini hand" is still sometimes discussed to describe some visible symptoms in the hands.
He got syphilis relatively young, and was treated (read: not treated at all) with mercury. He also got Tuberculosis which wasn't ideal, but even on his deathbed he didn't realise how sick he was. (Classic chronic illness.) He refused the last rites because he thought he didn't need them yet, so when he died without them, it didn't help the whole "sold your soul to the devil" thing.