FOR Dr. Philip M. Tierno, business travel means combat. The enemy is heartless and relentless, stealthy and ubiquitous. It is poised to attack every second of the day, even when he’s asleep. It exploits his rare moments of weakness and turns everyone and everything around him into agents who thoughtlessly do its bidding.


Dr. Tierno is not a corporate security expert trying to outflank rogue competitors. His enemy is infectious disease. He’s a professor of microbiology at the New York University School of Medicine who sees the common places of travel — cabs, airports, airplanes, hotels, restaurants and business meetings — as battlefields in the never-ending war against germs.


“Fecal organisms, oral bacterium, respiratory flora and thousands of other pathogens are here, there, everywhere,” Dr. Tierno says with studied calm.


Of course, you don’t need to travel to encounter germs. They’re all over your home and office. They’re lurking at the supermarket and the pharmacy. They may be on your hand right now. A Yale researcher recently found that a person entering a room could add about 37 million bacteria to the air every hour. Still, business travel poses special challenges like increased exposure to germs, the long-term health effects of the road warrior lifestyle and the difficulties of finding and paying for quality care away from home.


Getting sick is basically a numbers game. Eighty percent of all infectious diseases are transmitted by direct or indirect human contact. When we travel, our encounters skyrocket. When you lay your hand against the seat of a cab to pull yourself in, it is as if you are shaking hands with every rider since the cab was last cleaned (a concept best measured in geologic time). Gripping the airport escalator hand rail is like holding hands with thousands of fellow passengers. Your close encounters of the infectious kind continue on the plane as you touch the armrest of your seat, the tray table, in-flight magazine and the lavatory faucet and door handle. And your hotel? Depending on when and how it was cleaned, you may not be sleeping and showering by yourself.


Now consider that only about half of us wash our hands after going to the bathroom. It’s enough to make you echo a line from David Foster Wallace’s novel, “Infinite Jest”: “Yes, I’m paranoid — but am I paranoid enough?”


The link between travel and illness may seem self-evident but there is little hard data precisely defining the risk. A wealth of research has documented the swarming mass of microscopic meanies that hitch a ride on us as we travel. Scientists have swabbed everything from taxi cabs to hotel remote controls for analysis. Outbreaks of influenza, measles, severe acute respiratory syndrome, or SARS, tuberculosis and smallpox have been reported on commercial airliners since 1946. The Sept. 11 attacks provided a tantalizing piece of evidence. The flu season arrived 13 days later than normal in 2001, perhaps because the government shut down the air transport system (a k a the germ transport system).


More broadly, a study of data collected from passengers flying between San Francisco and Denver found that travel increased their likelihood of getting sick by 20 percent. “It seems clear that the full range of experiences we have while traveling, from the time we leave our house, to the time we return, makes it more likely we’ll come down with something,” said Dr. Mark Gendreau, an expert in travel and aviation medicine based in Massachusetts.


The one bright spot is the lack of evidence to date that airport security poses any risk of radiation poisoning.


Before you shut your windows, lock your doors, cancel your Expedia account and download videoconferencing software, remember that all these risks can be mitigated. Unless you have an open wound, the germs you collect on your hands cannot enter your body easily unless you touch your eyes, nose or mouth. That is easier said than done; we touch our mouths and noses about 200 times a day. A good cleaning with soap and water — the Centers for Disease Control and Prevention recommends washing your hands for about 50 seconds, the time it takes to hum two choruses of “Happy Birthday” — or a drop of hand sanitizer does the trick in most cases. But that only wipes the slate clean, providing little protection after that moment. Seasoned travelers sanitize early and often. (Researchers pooh-pooh the idea that too much cleanliness will weaken an adult’s immune system. The bottom line: That which does not kill me can make me sick.)