Excerpt from Terry Pratchett's Hogfather (p. 172):
"What's going on?" said Ridcully, as the others pushed in behind them.
"I know it sounds stupid, Archchancellor, but we think it [a malfunctioning magical computer] might have caught something off the Bursar."
"Daftness, you mean?"
"That's ridiculous, boy!" said the Dean. "Idiocy is not a communicable disease."
Ridcully puffed his pipe.
"I used to think that, too," he said. "Now I'm not so sure. Anyway, you can catch wisdom, can't you?"
"No, you can't," snapped the Dean. "It's not like the flu, Ridcully. Wisdom is ... well, instilled."
"We bring students here and hope they catch wisdom off us, don't we?" said Ridcully.
"Well, metaphorically," said the Dean.
"And if you hang around with a bunch of idiots you're bound to become pretty daft yourself," Ridcully went on.
"I suppose in a manner of speaking ..."
"And you've only got to talk to the porry old Bursar for five minutes and you think you're going a bit potty yourself, am I right?"
The wizards nodded glumly. The Bursar's company, although quite harmless, had a habit of making one's brain squeak.
"So Hex [the magical computer] here has caught daftness off the Bursar," said Ridcully. "Simple. Real stupidity beats artificial intelligence every time."
... which goes a long way to explaining why I get more daft the more freshman comp papers I grade. Perhaps I should wear latex gloves.