Flying to great-aunt Erma's house for Thanksgiving? Or taking a leisurely wintertime cruise along the shores of the Mexican Riviera? Boost your chances of healthy travel by taking a few preventive steps. That way, you'll cut your risk of catching cold and flu from other plane passengers. And you won't be confined to your cabin on the cruise ship, battling a nasty case of gastroenteritis while other passengers are off enjoying the sights.