Sadly, this will not surprise you.
A new British study found that riding the subway greatly increases your risk of respiratory infections. From The Daily News:
The study followed 138 patients at a doctor's office, and asked them whether they had ridden a bus or tram recently.
Far more of those who used public transportation in the previous five days reported getting sick than those who didn't, the University of Nottingham study found.
So cramming into a poorly ventilated box full of sick people will increase your exposure six-fold to various viruses? Yes. But there is an upside for daily commuters: riding the subway regularly will increase your immunity to these nasty subway viruses. So basically, it's only the fair-weather riders who are more at risk (sorry to those of you lugging ten Century 21 and Abercrombie & Fitch bags).