FRIDAY, Jan. 21 (HealthDay News) -- People who use public transportation have a nearly sixfold increased risk of developing an acute respiratory infection that requires a visit to the doctor, researchers report.

But, people who travel by public transit every day might be somewhat more protected from acute respiratory infections than occasional users of the transit system, the researchers at the University of Nottingham in England found.

The study was conducted in Nottingham during an influenza outbreak between Dec. 2, 2008 and Jan. 15, 2009. The researchers asked 138 patients (72 with an acute respiratory infection and 66 controls) at a doctor's office about their use of public transit in the five days before the start of respiratory symptoms or before their medical appointment.

"We found a statistically significant association between acute respiratory infection and bus or tram [streetcar] use in the five days before symptom onset. The risk appeared greatest among occasional bus or tram users," Jonathan Van Tam, a professor of health protection in the School of Community Health Science and director of the Health Protection Research Group, said in a University of Nottingham news release.

"These data are very plausible when we think about the greater likelihood of developing protective antibodies to common respiratory viruses if repeatedly exposed. The findings have differing implications for the control of seasonal acute respiratory infections and for pandemic influenza. In the latter case we don't have an opportunity to build up any immunity beforehand because it's, by definition, a new virus," Van Tam said.

The study was published in the Jan. 14 online edition of the journal BMC Infectious Diseases.

More information

The American Academy of Family Physicians has more about colds and the flu.