Ordinarily, getting on a plane isn't reason enough to be detained, but for Andrew Speaker, a then 31-year-old lawyer from Atlanta, it was. Infected with a nasty form of multi-drug-resistant tuberculosis (MDR TB), Speaker disobeyed health officials — and potentially exposed hundreds of people flying with him — when he boarded a commercial plane headed from Prague to Montreal this past spring. Despite being exposed, the passengers so far seem to have escaped infection. But his case revealed just how easy it is for dangerously infectious diseases to spread and how difficult such pathogens are to control in a world in which globe-hopping passengers are such willing couriers for bugs hitching a free ride.