“There’s no U.S. requirement that someone announce to the airline authorities that they have a potentially contagious disease or even answer a question about it, even if asked,” said Jeffrey Ment, an expert on travel law.

The cruise industry requires passengers to submit to a health screening before boarding to prevent outbreaks of norovirus, and passengers are told to expect the questions before they arrive, said Ment, a Hartford, Connecticut, lawyer.

Airlines, by contrast, have no such rule, though they are required by U.S. regulations to deny boarding to passengers they believe are sick and would pose a danger to others on a flight.