GENEVA, Switzerland - The World Health Organization marked a grim milestone Thursday in the biggest-ever Ebola outbreak, estimating that the virus had killed more than 10,000 people, mostly in the West African nations of Guinea, Liberia, and Sierra Leone.

Fifteen other Ebola deaths also occurred in Mali, Nigeria, and the United States.

When Ebola was first detected in March 2014 in Guinea's forest, officials assumed the deadly virus could quickly be stamped out, just as it had in more than two dozen previous outbreaks, mostly in central and eastern Africa. But health officials now acknowledge they were too slow to respond to this emergency, allowing Ebola to cross porous borders in a region where broken health systems were unable to stop its spread.

A huge global response - including soldiers sent by Britain, the United States, and other nations - has slowed the deaths from Ebola dramatically, especially in Liberia, but the virus appears stubbornly entrenched in parts of Guinea and Sierra Leone.

WHO declared Ebola an international health emergency in August - but critics have slammed the agency for waiting until there were nearly 1,000 deaths to do so. WHO recently announced it was forming an independent expert panel to assess its response. Ebola cases also emerged elsewhere in Africa, including Nigeria, Senegal and Mali, and small outbreaks were later identified in the U.S. and Spain.

Liberia, once the hardest-hit country in the Ebola outbreak, released its last Ebola patient on March 5. It has now begun a 42-day countdown, and if no new cases are found in that period, it will be declared Ebola-free according to WHO standards.

Meanwhile, the National Institutes of Health announced Thursday that an American health worker who contracted Ebola while volunteering in Africa will be admitted to a hospital at the agency.

The patient was expected to arrive Friday at the NIH research hospital in Bethesda, Md., after being transported to the United States in isolation on a chartered plane. The patient's name, age, and gender have not been released.

The patient had been volunteering at an Ebola treatment center in Sierra Leone. The NIH did not release any further details about the patient.

The agency has one of the few specialized isolation units nationwide that are set up to treat Ebola patients. Previously, an American nurse was treated there after she contracted Ebola while caring for a Liberian man who died at a Dallas hospital. The nurse, Nina Pham, survived and is Ebola-free.