Reviewing an answer on another question, someone posited that Professor X is a mutant, and thus not subject to normal rules of mortality. This isn't really true, but it brought up an interesting idea, and after wracking my brain, I can't come up with a single example.
Within the Marvel or DC universes, has any superhero died of natural causes (old age, natural disease, etc)?
For purposes of this question, let's limit this to primary continuities/universes (The Watchmen was published by DC, but don't exist within the main DC universe). Being killed in a battle, or from injuries sustained in a battle would also not count.