In what the World Health Organization called a "monumental achievement," Liberia has halted its Ebola outbreak.
But the organization cautioned that Ebola persists in neighboring Guinea and Sierra Leone, creating a "high risk" that the virus might spread back into Liberia across porous borders.
The medical aid group Medecins sans Frontieres (Doctors Without Borders) urged improved cross-border surveillance to prevent the disease from re-emerging in Liberia.
"The Liberian government and the Liberian people have worked hard to achieve 42 days of zero Ebola cases, but that hard work could be undone in an instant," Mariateresa Cacciapuoti, the organization's head of mission in Liberia, said in a statement.
"We can't take our foot off the gas until all three countries record 42 days with no cases."
Last week, Guinea and Sierra Leone each reported nine confirmed cases of the virus, the lowest numbers this year.
The Liberian outbreak officially ended Saturday, 42 days after the last known Ebola patient died. The period is chosen because it is twice the incubation period of the virus; if no new cases appear, experts assume that all transmission chains have been interrupted.
The last case was a woman in the Monrovia area who developed symptoms March 20 and died seven days later. Officials identified and monitored 332 people who might have been exposed to her; none developed symptoms and all are now free of surveillance.
Overall, the toll in the epidemic has been huge -- some 26,722 confirmed, probable, or suspected cases in the three West African nations and handful of cases elsewhere, including four diagnosed in the U.S.
That number includes 11,064 deaths, the WHO reported Monday, including 4,769 in Liberia, which had the highest number of fatal cases.
When Ebola transmission was at its peak -- August and September, 2014 -- Liberia had between 300 and 400 new cases weekly. While Monrovia, the capital, was hardest hit, no part of the country was spared with the case total eventually reaching 10,604.
The spread of the virus was exacerbated by travel among the three countries, which share mutual and relatively uncontrolled borders.
The outbreak began in Guinea late in 2013 but was not recognized until March 23, 2014. The first cases in Liberia appeared a week later, in a region bordering Guinea.
The first recognized case in Sierra Leone occurred in late May; a woman admitted to hospital after a miscarriage was found to be positive for Ebola, was isolated, and later recovered.
But investigation linked the young woman to an earlier event that illustrates the cross-border flow of the virus, according to the WHO.
The region of Kenema was home to a traditional healer, whose powers were also known across the border in Guinea. As the outbreak in Guinea grew, patients crossed the border to seek care -- quickly infecting the healer herself.
When she died, the WHO reported, hundreds of mourners took part in a traditional funeral and burial ceremony -- and local health authorities think participation in that event could be linked to as many as 365 Ebola deaths.
With the epidemic now apparently winding down, attention is turning to what has been learned.
The World Health Assembly, the decision-making body of the WHO, is to meet next week to discuss -- among other things -- an interim report on what went wrong and what went right.
The epidemic, the report says, "the largest and most complex Ebola outbreak on record."
It was "particularly challenging," the report says, because of:
- The characteristics of the virus itself, which spreads by contact between people and found fertile ground in the traditional hands-on burial and funeral practices of the region
- Fragile healthcare systems and public health infrastructure in the region, which had "structural weaknesses, inadequate surveillance, and few laboratories"
- Underlying mistrust of governments within the communities
- Highly mobile populations that easily crossed and re-crossed porous borders
- The "urban context" of the epidemic
Previous outbreaks of the virus had taken place in isolated rural areas and rarely had more than a few hundred victims. Importantly, the virus had not previously been seen in West Africa.
The report also says it's not completely clear why early warnings "did not result in an effective and adequate response."
One of the key issues, the report says, is that the WHO does not have the capacity to launch an emergency response on its own on a scale that would have stopped the outbreak more quickly.