The Master shook his head, barely keeping back a sigh of his own.

"You really are a silly, naive child," he scolded. "The older Doctor may be from your future, but not necessarily from your reality. If you're wise, you'll tell every version of the Doctor you can find. Think about it, my dear. The younger Doctor has been here for years and this Castle, apparently, can accomplish wonders. He may be able to use it to find knowledge of this virus and, from there, a vaccine. In any case, it is better to try, than to do nothing."