How do you hold in a cough for so long?

Mike Mennonno took his first subway ride in awhile yesterday - armed with a copy of an E.B. White essay he wanted to read about a dying pig with an infectious disease (Mike is cheery like that). He reports an amazing thing:

... [N]o one -- and I am not exaggerating here -- no one in my car coughed or sneezed the entire trip. Seriously, I don't think I've ever experienced anything like it before. Usually people do a good deal of coughing and sneezing on public transit. I've always suspected that most of it is psychogenic -- no physical cause. Rather, people do it to further insulate themselves when in close contact with strangers. They're essentially saying: "Stay away! I'll cough on you! Keep your distance! I'll sneeze on you!" Not that they're exactly conscious of their motives. ...