UPDATE 10:00 a.m. EST Friday: A dead woman’s body tested positive for Ebola in Sierra Leone just hours after the World Health Organization declared the region Ebola-free on Thursday, underscoring the need for vigilance in the fight against the deadly epidemic.

Mariatu Jalloh, a 22-year-old student – who died on Jan. 12 – began showing symptoms at the start of the year and may have been exposed to at least 27 more people, Reuters reports. She went to receive treatment on Jan. 8 at a local hospital, where she was treated as an outpatient then sent home. She also traveled around Sierra Leone in December, even making a stop in a town close to the Guinean border. Sierra Leone was declared Ebola-free on Nov. 7.

FREETOWN, SIERRA LEONE and JOHANNESBURG, SOUTH AFRICA --- Since early December, public health officials around the world held their breath as the days quietly stacked up: first a week without a new Ebola case in West Africa, then another, then another. But their optimism remained guarded in case the disease suddenly resurfaced.

Thursday, however, marked a new and momentous threshold – 42 days, or twice the disease’s incubation period, without an active case anywhere. That was enough for the global health community to make a long-awaited declaration: Two years, ten countries, and 11,000 deaths later, the world’s worst Ebola outbreak was officially over.

“We don’t want to say ‘Ebola is gone,’” says Dr. Alex Gasasira, director for the World Health Organization office in Liberia, the last country to have an active Ebola case. “Rather, we are declaring that this particular epidemic appears to be over. But that doesn’t mean it won’t come back, so we must continue to be prepared and to be vigilant.”

​Now, the region faces its greatest public health challenge: to transform the lessons of ​the epidemic​'s darkest days into long-term public health gains. ​​This means addressing the cracks in public infrastructure, local governance, and international public health response​- and making to keep those systems from buckling again.

One crucial lesson that applies to all infectious diseases is a simple one, says Axelle Ronsse, an Ebola emergency response coordinator for Doctors Without Borders (MSF): Get there faster.

The sluggish response by both local governments and the international community to the first reports of confirmed Ebola cases in early 2014 allowed the disease to slip unnoticed over the border from rural Guinea into neighboring Liberia and Sierra Leone, and from there on to overcrowded cities.

“One key lesson … is that we are dependent on each other and disease has no borders,” says Dr. Foday Dafae, director of disease prevention and control in the Sierra Leonean Ministry of Health. “Equally so we need to train our work force to be prepared at all times to be able to respond as soon as there is an outbreak.”

Enemy or ally?

But even when Ebola treatment centers did open, they initially struggled to attract patients; to locals, they were little more than places where the desperately sick went to die. A flurry of military-led quarantines and curfews compounded the paranoia, convincing many Guineans, Sierra Leoneans, and Liberians that the healthcare system was enemy rather than ally.

The turning point, many experts say, came only when populations began to hear messages on how to prevent and treat Ebola not from doctors and national governments – distant and imposing – but from those they trusted most: priest and imams, chiefs and village elders. In one notable case, a famous Sierra Leonean comedian was the trusted voice. Only then did communities begin to give up the gestures of love and compassion, such as washing the bodies of the dead, on which Ebola, cruelly, thrived.

At the epidemic’s height in late 2014, nearly 1000 cases per week were being reported across the three countries. By mid-2015, the number of cases had fallen sharply; on May 9, Liberia became the first of the three countries to declare itself Ebola free.

But by June, three new cases had surfaced in Liberia. The disease then reappeared again in November. Guinea was declared Ebola-free on Dec. 29.

All three countries say that in future they will focus on early detection of Ebola and other infectious diseases.

Gasasira, in Liberia, says the country is developing a network of community early-responders to report any suspicious illnesses. And Dr. Dafae, in Sierra Leone, says his unit now conducts regular monitoring for 47 diseases, including yellow fever, around the country. “The situation calls for vigilance,” he says. “We as health authorities are saying complacency is not the word.”

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For Aminata Kamara, a petty trader in Freetown, Sierra Leone, the end of an epidemic on Thursday that killed several of her friends and relatives was greeted with muted excitement.

“I am happy it is all over and we are back to normalcy,” she says.