If you are concerned that a nearby passenger has tuberculosis -- or, in fact, any respiratory infection -- you might do best to try to move as far away as possible.
"You generally have a range of about three feet or so [for transmission of viruses]," Freedman said. "Even on airplanes, many people blame the ventilation system and things like that, but really it's who you're sitting next to. We call it the 'two-seat' rule."
"If you are sitting next to someone who is coughing and you are worried about tuberculosis or another infectious disease, you can ask to change your seat," Schaffner suggested.
But barring that, he said, you might want to ask the sick person beside you to use a tissue to cover their coughs.
A tissue? Don't laugh. Schaffner said that in the age of tuberculosis sanatoriums, patients who were institutionalized for their infections were first taught to cover their coughs with a tissue. This, apparently, was enough to prevent those in the sanatorium who were not infected with the bacterium from catching it.
The next time you find yourself packed into a crowded train or bus during your vacation, you might try not to think too much about the germs you could be breathing in -- particularly for the cold and flu.
"Most of these illnesses are transmitted within a breathing space," Schaffner said -- which means that the flu virus that your traveling neighbor has may soon be yours if you aren't careful.
So what is the best advice to avoid an infection? As with the seating on an airplane, location is everything.
"Get somewhere and sit somewhere else," Dr. Howard Markel, professor of the history of medicine and of pediatrics and communicable diseases at the University of Michigan Medical School, told ABCNews.com in September. "If someone is coughing, I would avail myself that opportunity. Our mothers were right when they said, 'Don't let anyone cough on you.'"
More often than not, however, trains and buses are so packed that it's difficult to remove yourself from such situations. If you find yourself trapped, it may be best to try and limit your exposure as much as possible.
"Breathe shallowly," Markel recommended. "Try not to let [people] cough on you."
In that way, he explains, transmission of germs can be reduced.
As on cruise ships, the surfaces that you touch can also harbor some nasty bugs. However, Schaffner said, these surfaces are less likely to be a source of viral transmission in these cases. With this in mind, he said, people should still be practicing safe hand hygiene if they are in such situations.
A commonly known nuisance, bedbugs are known for the red, itchy bumps that their bites leave on the skin of unsuspecting travelers.
According to reports from the U.S. Public Health Service, bedbugs are known to carry dozens of infectious diseases, from smallpox to the flu.
Last May, Edward Brownbear, lead education instructor for the Department of Housing, Preservation and Development (HPD) in New York City, told an audience at a seminar that the city's trains and stations may be infested with bedbugs.
Brownbear reportedly said he had seen the bugs on the wooden benches of Manhattan's Union Square station and The Bronx's Fordham Road station -- as well as on the clothing of a passenger on a train.