AKA Economy Class Syndrome. This is blood clots that form when a person is forced to sit immobile for a long period of time. Clots in the brain, heart, and/or lung are, of course, serious.
A study in Tokyo's Narita Airport has shown that 25 people have died from the problem in the airport clinic in the past 8 years. (Insert Airport Terminal joke here.) About 150 had serious, though not fatal, other problems.
Source: Reuters