— Despite stringent infection-control measures, the risk of Ebola’s spread cannot be entirely eliminated, Doctors Without Borders said Friday after one of its doctors caught the dreaded disease while working in Guinea and went to New York City.

In another example of how the disease has jumped borders despite increased health checks at land crossings and airports and seaports, Mali reported its first confirmed case of Ebola late Thursday. Authorities in the West African country are monitoring 43 people, including 10 health workers, said Fadela Chaib, a spokeswoman for the World Health Organization in Geneva. The organization is sending experts to the country to help assess. A 2-year-old girl who came to Mali from Guinea was the confirmed Ebola case.

The Ebola outbreak in West Africa was first identified in March in Guinea and quickly spread to Liberia and Sierra Leone. It has sickened nearly 10,000 people, the vast majority of them in those three countries, but cases have also been reported in Senegal, Nigeria, Spain and the United States. Mali, which borders both Senegal and Guinea, was long considered highly vulnerable to Ebola’s spread.

Because symptoms of the disease can take up to 21 days to appear, people can travel long distances before they know they are infected, bringing the disease to a new place.

The American doctor who was working in Guinea for Doctors Without Borders arrived back in New York about a week ago and reported that he had a fever on Thursday. Craig Spencer is now being treated for Ebola in a hospital in New York.

“Contagion is everywhere,” said an official in Guinea for Doctors Without Borders, also known by its French name Medecins Sans Frontieres or MSF. “Even in Conakry, the risk isn’t zero. But MSF takes measures to secure its personnel.”

She refused to say where or how long Spencer had been working in Guinea. The official spoke on condition of anonymity because of the sensitivity of the situation.

Doctors Without Borders, which has been treating patients since the outbreak was identified in March, runs two treatment centers in Guinea: one in Gueckedou, in the country’s southeast, where the outbreak began, and the other in the capital, Conakry. In Macenta, near Gueckedou, it also runs a transit center where patients are screened for Ebola and then sent on for treatment if they are confirmed to have the disease.

With 270 international staff and more than 3,000 local staff working in the region, Doctors Without Borders is a major provider of treatment in the outbreak.