And while this is an untested theory I would assume that poor hygiene plus physical exertion, especially if combined with ill-dressed outdoor activities in winter, should help bring on a cold/flu.
(edited to clarify a teeny point)
That seems unlikely. I've sort of been testing your theory unwittingly. I'm playing the Apocalypse challenge, neighborhood still set to Winter-Winter-Winter-Winter because I haven't lifted Music yet. Thus, my sims get to sponge-bathe outside because I sadistically placed the outdoor sink (CC that looks like a bucket) on the balcony. Not-so-clean-to-start-with sims get stinky working out to the boombox, then go sponge off starkers in a raging snowstorm. And their thermometers do drop into the blue while they wash in that delightfully cold water.
Not a one has gotten sick. I'm still only in the second generation, so I will watch out for any evidence of sickness. I really think sims need to come in contact with a disease vector or ingest spoiled food in order to get sick.
I suppose the most acid test would be to let a sim get filthy, then have it go swimming outside in a snowstorm. I think I'll try that.
Edited to add:
Blargh. Stupid EAxis. A sim shouldn't be able to swim for two hours in a snowstorm and not get cold. I guess the pools are heated? Bah.
But the sim got cold in a hurry once I had him get out of the pool. It's late so I didn't wait to see if he would come down with a cold. More investigation is warranted after I get some sleep.