A case of measles reported in Hennepin County last month has not caused other infections — ending fears that it would set off a larger outbreak, state health officials reported Tuesday.

But just as they were about to declare the all clear, another unrelated case popped up last week, also in Hennepin County. Like the earlier case, this one involves an unvaccinated child who had traveled here from Africa and brought the infection along.

The cases underscore an international surge in measles, including an outbreak in Europe, that has health officials concerned in Minnesota. Even though the case from early August was contained and it appears that few people have been exposed to the new case, the deepening pool of infections worldwide has the potential to spark an outbreak in the United States among the pockets of children and adults who remain unvaccinated.

“You just never know when somebody is going to bring back measles,” said Kris Ehresmann, infectious disease director at the Minnesota Health Department.

Because measles has been eradicated in the United States, all new cases here are imported from other countries, where the highly contagious virus can circulate easily among those who are not vaccinated or lack natural immunity.

Last year’s Minnesota outbreak, which sickened 75, also got its start as an import, although health investigators were never able to identify the source.

Altogether, that outbreak sent more than 20 people to the hospital and cost state and local governments more than $1 million because of a complex and ambitious public health effort to contain it.

Even now, the constant threat posed by measles has hospitals and clinics screening patients carefully for those who have traveled internationally recently.

“Measles is the most contagious viral illness that we have,” said Patsy Stinchfield, senior director of infection prevention and control at Children’s Minnesota, which treated both of the recent cases.

The viral particles that make up measles are light and float easily in the air. By comparison, influenza is somewhat less contagious because its viral particles sink to the ground, Stinchfield said.

“With flu you need pretty close contact,” she said. “With measles it doesn’t take much.”

In addition, measles can linger in the air for two hours in a room where an infected person has been. Thus, a traveler could contract measles simply by passing through an airport that was recently visited by an infected passenger.

That’s why Children’s asks people if they have traveled anywhere outside the United States, even if it was not a country where measles is endemic. Incoming patients who have traveled abroad are immediately taken from the hospital’s waiting room into a separate room with negative air pressure, which keeps any viral particles from circulating to areas with patients, many of whom have weaker immune systems. In the hospital’s emergency department, air is exchanged at a fast rate and is run through filters and an ultraviolet light, which can help kill pathogens.

Minnesota’s most recent case is a 2-year-old child who lived in Kenya and traveled to Minnesota for specialized medical care unrelated to measles. The child’s measles symptoms became apparent on Aug. 20.

“Thankfully the individual was not out and about with the general public, so there is minimal risk there,” Ehresmann said. Measles can sometimes take eight to 10 days to develop, but epidemiologists conservatively wait three weeks to see if other cases develop before issuing an all-clear. The clock on the second case will run out on Sept. 14.

The biggest concern, health officials said, is that an infected person will cross paths with someone who is unvaccinated.

“Our state immunization rate is relatively high, but there are pockets of unimmunized individuals all over Minnesota,” Stinchfield said. “This is not just a Hennepin County or newcomers issue.”

The Somali-American community has been particularly vulnerable in recent years because measles vaccination rates for its 2-year-olds fell to about 42 percent after autism fears and distrust of medical professionals took hold.

Concerted outreach efforts during last year’s outbreak boosted the rate to 59 percent.

“We are encouraged that there was a little bit of an increase, but we want to be clear that immunization rates need to be in the 90 to 95 percent range to prevent an outbreak,” Ehresmann said.

The number of measles cases in the United States has grown steadily in recent years, from 86 in 2016 to 118 in 2017 and 124 so far in 2018. Worldwide, the number of known cases has jumped 55 percent, to 130,000, so far this year.