We did not find an increased risk of influenza in the 58% of participants who were regular users of public transport. Public transport usage has been cited as a possible risk factor for influenza infection, particularly in the context of pandemic planning [20] . In a recent international survey of precautionary behaviour for pandemic influenza, 75% of respondents said that they would avoid public transport [21] . Both rail and air travel have been associated with respiratory infection transmission [22, 23] . Whilst it is possible to contract influenza on public transport, our results suggest that, at least in Berlin, using public transport does not increase a person's risk of influenza. In Berlin, crowding on public transport is infrequent and not extreme, so this result may not be generalisable to cities with different conditions.