A study in the U.K. found that the use of public transportation (bus or tram) is a risk factor for acute respiratory tract infection and that frequent users of public transportation have lower risk of contracting acute respiratory tract infection than others. The researchers found that patients of acute respiratory tract infection are six times more likely than people in the control case to have traveled on bus or tram within 5 days before symptoms onset.
An interesting finding from this study is that regular users of public transportation (>3 uses/week) are at lower risk of contracting acute respiratory tract infection than infrequent users (<3 uses/week). One plausible explanation is that regular commuters are more frequently exposed and may have acquired protective immunity.
So for those of us who ride the public transportation to work every day, consider ourselves lucky!