I think I’ve only read one book with a sick dragon. I could be forgetting something, but I’m fairly confident of this. In Dealing with Dragons, the dragon Kazul states that, if something isn’t going to kill a dragon in the first few minutes, it’s just not going to kill the dragon. However, dragonsbane, a purple weed, can kill them—if the taste can be disguised well enough to get a dragon to eat it.
There’s one other thing in Dealing with Dragons that makes dragons sick—wizard’s staffs. Dragons are allergic to the magic wizards use, which is stored in their staffs. So, when one of the dragons is overexposed to a staff, he gets a sneezing fit of epic proportions. This happens more than once within the story.
In most books, this is the case: that very few things can easily kill a dragon. A sword, magic, a magic sword—these are all common things needed for killing dragons. Granted, they’re usually considerably larger than their human opponents, but the fact remains that they’re much better armored.
In Eragon, killing a dragon’s rider will kill the dragon. This one never sat right with me, since dragons are so much larger and more powerful than humans (excepting sorcerers). I couldn’t quite get over the physical strangeness of it. Still, it is an interesting concept. It also does serve as a refreshingly simple way to kill what is usually considered to be the unkillable.
In several different series—Dragonriders of Pern, Dragon Slippers, and Dragon Jousters—dragons are seen to fight and wound or kill one another. That makes sense, considering people are able to hurt each other, lions the same, and any other species you can think of.
What you never see is anything other than flame, steel, or claw wound them. Magic? Yes. Arrows? Definitely.
A cold? Never. An anvil falling on their foot? No again. The plague? Not a chance.
Prove me wrong.