Demon pox. There's always demon pox.

Demon pox, oh demon pox
Just how is it acquired?
One must go down to the bad part of town
Until one is very tired.
Demon pox, oh demon pox, I had it all along—
Not the pox, you foolish blocks,
I mean this very song—
For I was right, and you were wrong!"

"Will!" Charlotte shouted over the noise, "Have you LOST YOUR MIND? CEASE THAT INFERNAL RACKET! Jem—"
Jem, rising to his feet, clapped his hands over Will's mouth. "Do you promise to be quiet?" he hissed into his friend's ear.
Will nodded, blue eyes blazing. Tessa was staring at him in amazement; they all were. She had seen Will many things—amused, bitter, condescending, angry, pitying—but never giddy before.
Jem let him go. "All right, then."
Will slid to the floor, his back against the armchair, and threw up his arms. "A demon pox on all your houses!" he announced, and yawned.
"Oh, God, weeks of pox jokes," said Jem. "We're in for it now.

Demon pox, oh, demon pox,
Just how is it acquired?
One must go down to the bad part of town
Until one is very tired.
Demon pox, oh, demon pox I had it all along—
No, not the pox, you foolish blocks,
I mean this very song—
For I was right, and you were wrong!

Gabriel’s green eyes sought Will. “It was demon pox, wasn’t it? You know all about it, don’t you? Aren’t you some sort of expert?”
“Well, you needn’t act as if I invented it,” said Will.

Will: Have you ever seen what happens to someone with demon pox? First it lies dormant. One begins to turn yellow and green. Then the swelling sets in -
Jem: THERE IS NO SUCH THING AS DEMON POX.

Oh, leave it,” said Jem, kicking Will, not without affection, lightly on the ankle.
“She’s annexed my plan!”
“Will,” Tessa said firmly. “Do you care more about the plan being enacted or about getting credit for it?”
Will pointed a finger at her.
“That,” he said. “The second one.