Wisdom a Communicable Disease?

‘What’s going on?’ said Ridcully, as the others pushed in behind them.
‘I know it sounds stupid, Archchancellor, but we think [Hex] 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 sure. Anyway, you can catch wisdom, can’t you?’
‘No, you can’t,’ snapped the Dean. ‘It’s not like ‘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.

(from Hogfather by Terry Pratchett)