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With 650 million tourists crossing international borders annually, travel is a leading cause of unintended invasive-species introductions. Fungi, insects and seeds hitch rides in luggage, on food and clothing, even on our bodies. "The organisms...that we import intentionally pale in numbers beside the masses of smaller living things we set in motion incidentally," cautions Yvonne Baskin in A Plague of Rats and Rubbervines: The Growing Threat of Species Invasions. And not without cost: In the U.S., invasive species have contributed to the decline of up to 46 percent of imperiled species.

Some countries require aircraft "disinsection" to crack down on alien invaders, forcing airlines to spray cabins with insecticides, including synthetic pyrethroids, which can spark asthma attacks. Passengers may be subject to in-flight spraying on some flights to Asia, Africa, the South Pacific and the Caribbean, while other countries spray planes immediately before passengers board.

What You Can Do

- Thoroughly clean boots and gear after use and avoid transporting seeds, fruits or other natural souvenirs.

- Ask if disinsectants will be sprayed on your flight, and request to deplane beforehand.

- For countries requiring aircraft disinsection, see the U.S. Department of Transportation.

In the end, if the litany of pitfalls awaiting your travels seems daunting, consider that most travelers return home healthy and happy, with memories of a lifetime. And—with planning and a little presence of mind—you should, too.