Doc Q, the doctor of Blackbeard pirates, seems to be a very weak man. He's probably ill, but the fact he's a doctor puts on him a ridicolous aura. I was thinking he has a secret power, a DF or an extraordinary ability, allowing him to join the most bastard crew in One Piece's world. My new idea is instead that he's simply a great doctor, as Hogback before his alliance with Moria or maybe Doctorine. And following this idea, why doesn't he heal himself? My conclusion is: doc Q has the same fatal disease that took Roger. That illness is probably rare but not unique; even in real world no doctor can heal some thing as cancers. Doc Q, as Roger, shows moments of pain and other moments of normal attitudes. Actually we didn't see nothing of Roger but i'm sure he suffered sometimes, while we see him normally behaving during, examples, edd war or his execution.
I have no proof to support my idea but i hope you'll enjoy my speculation.