Saw an unusual sight on my first flight since the idiot lawyer from Atlanta flew to and from Europe despite his TB infection: a guy in front of me wore a surgical mask the entire trip:
It could be he has a pre-existing condition, but my guess is this fellow feared risk of TB. That, of course, would be innumeracy: the odds that someone as selfish and stupid as Andrew Speaker are next to you on the plane are infinitesimally small.
Back during SARS you saw a fair number of people in airports wearing masks. That wasn't innumeracy; the disease was highly communicable and did reach North America. If or when Avian Flu hits, it will be strange to see everyone walking the streets wearing masks. But strange sights will be the least of our worries.