I dunno, because PEOPLE GET COLDS??
And 'stuffed up' doesn't immediately mean 'cold'.
It could mean, oh, a reaction to someone smoking.
Ohh, I know. I have a cold because I've stuffed my head into a bucket of cold viruses, and plan to INFECT THE WORLD!
I mean, really, what else can I say?
"Why do you have a cold??" "I learned it from watching YOU!!"