A record 81 flights were investigated in 2018 for carrying at least one person contagious with the disease, up from 15 investigations in 2017 and 10 in 2016, according to the C.D.C. Of the 106 aircraft, 66 were on domestic routes and the rest were inbound from foreign airports.

Because the United States has a fairly high overall vaccination rate against measles, the general population is largely protected by herd immunity. But there are states, and pockets within states, with far less compliance, and the concern is that infected fliers will transport measles to these vulnerable areas.

“Suddenly the single introduction of a case can have explosive consequences,” said Dr. Martin Cetron, the director of the Division of Global Migration and Quarantine at the C.D.C.

Measles is considered among the most contagious viruses in the world. Unlike the influenza pathogens, which spread when a person sneezes or coughs but then fall, the measles virus remains suspended in air, “like really light dust particles,” Dr. Cetron said, allowing the pathogens to linger and float for up to two hours.

Influenza is spread by an exponent of two, meaning each person who gets it is likely to infect two others in a setting where people haven’t been immunized, he said. Measles is likely to be spread to between nine and 19 people who are not immunized, depending on the setting and the herd immunity of those nearby, he said.