WASHINGTON — A crucial issue in fighting the spread of the coronavirus is ensuring that countries with travel-related cases of the virus, like the U.S., take steps to prevent a spread that cannot be contained, the head of the National Institute of Allergies and Infectious Diseases said Tuesday.

Speaking at a Council of Foreign Relations event, NIAID Director Anthony Fauci said the magnitude of the global infection is not known.

Coronavirus is highly transmittable and can leave people infected without showing symptoms strong enough to be detected on medical screenings.

“The critical issue now is how the countries that have travel-related cases are going to prevent them from being sustained transmissibility,” which means spreading beyond isolated clusters of people, Fauci said.

“Once you have sustained transmissibility, you have a real problem,”he said.

Fourteen infected Americans who were on a cruise ship that had been quarantined in Japan, were on two planes that flew them and other cruise ship passengers home Monday. Fauci said the United States has handled travel-related cases well, but there may be infected travelers who didn’t have symptoms when they flew. NIAID is going to send staff to visit clinics to test those with flu symptoms, but not the flu. Chicago, Los Angeles, Seattle, San Francisco and New York will be the first cities in this program.

Fauci said identification, isolation and contact tracing are the most important meansof getting the virus under control. If other countries are not able to do this, then the coronavirus outbreak will become a “classic pandemic,” he said.

The first case of the virus appeared early December in the Wuhan province in China, but Chinese officials did not announce the outbreak until January.

Since then, the Chinese government has taken several actions including commandeering the factory that creates N94 respiratory masks.

Those masks are essential to medical professionals in protecting themselves from contagious diseases when treating patients. China has stopped exports of the masks.

Robert Kadlec, the Health and Human Services assistant secretary for preparedness, stressed the need for more spending on biodefense budget and investment in domestic medical technology to protect citizens from current and future outbreaks.

“Public health security and national security are inseparable,” Kadlec said. Emerging disease threats are the next potential bioweapon, Kadlec said, and the government needs to prepare antibiotic stockpiles for the next potential outbreak.