A new Ebola case in Uganda is the first outside Congo since the outbreak began last summer. By Rick Gladstone

An Ebola treatment center in Beni, Democratic Republic of Congo. A new Ebola case in Uganda is the first outside Congo since the outbreak began last summer.CreditCreditBaz Ratner/Reuters

A 5-year-old Congolese boy who traveled with his family into Uganda has died from the Ebola virus, international health officials said on Wednesday — a day after the case was confirmed as the first appearance of the highly contagious disease outside the Democratic Republic of Congo since an outbreak began there a year ago.

The case, first reported by the World Health Organization on Tuesday, was identified soon after the child entered Ugandan territory. Nonetheless, it represented a new threat that the outbreak in Congo had crossed into a neighboring country.

By Wednesday, the World Health Organization said in a tweet that two more cases of the virus had been identified in Uganda; both patients are relatives of the boy who died.

The outbreak in Congo, the second largest on record, has infected more than 2,000 people and caused 1,390 deaths since it was discovered in July 2018. The number of cases has accelerated in recent weeks, health officials said.

Despite use of a new vaccine that appears to be effective, treating patients and tracking down those they may have infected have been persistent problems because the outbreak is in a chronic conflict zone in the eastern region of the Democratic Republic of Congo, a mineral-rich African nation that is three times the size of Texas.

Health workers, including doctors, have been attacked and killed, and some treatment centers have been destroyed. In April, the Islamic State claimed its first assault in the affected area.

In a statement, the World Health Organization said the infected child’s family entered Uganda on June 9 through the western Uganda border post at Bwera. They sought medical care for him at a hospital where workers suspected he had Ebola, which was confirmed by the Uganda Virus Institute on Tuesday.

Uganda has long prepared for the possibility of a spillover in the Congo outbreak. The Ugandan government has vaccinated nearly 4,700 health workers in 165 health facilities, including the one where the infected Congolese boy is being treated. Monitoring for signs of the disease has intensified.

Nonetheless, a recent spike in cases — and now the first cross-border case — could raise pressure on the W.H.O. to be more aggressive in its response to the outbreak.

“Now is the critical time to step up efforts to make sure that this one case does not become many,” Brechtje van Lith, Save the Children’s country director in Uganda, said in a statement. “The Ebola outbreak is already devastating lives and communities across eastern Congo — action and funds are needed right now to stop it spreading further across the region.”

A committee of outside experts that advises the director general of the W.H.O. has twice concluded that the outbreak does not represent a global health threat, partly because it has not spread across borders.

Declaring the outbreak as such a health threat, known as a Public Health Emergency of International Concern, would increase the level of attention that the W.H.O.’s member countries devote to combat it.

After the committee’s most recent conclusion, in April, its chairman, Robert Steffen, said the experts were “moderately optimistic” the outbreak could be brought under control. “Not immediately,” he said, “but still within a foreseeable time.”

Ebola is an aggressive viral disease that spreads through contact with the body fluids of infected people. It can lead to uncontrollable bleeding and death.

The largest Ebola outbreak in history ravaged the West African nations of Guinea, Liberia and Sierra Leone from 2014 to 2016, killing more than 11,300 people.