How will we know when an Ebola outbreak is really over?

Just one day after the World Health Organization declared Africa free of Ebola, Sierra Leone confirmed a woman died from the disease earlier this month.

Talk about a short-lived victory.

It wasn't the first time the WHO declared a county Ebola-free, only to see new cases.

New Ebola case confirmed in Sierra Leone

Liberia has been given the all-clear on Ebola three times, most recently Thursday. Although people who have fought the disease hope the virus is gone for good, health officials are holding their breaths and warning Ebola could flare up again. The WHO set a 90-day period for increased vigilance in Liberia, to make sure its epidemic is really over.

Some scientists say the world needs to stop declaring countries free of Ebola until they have a better understanding of the virus, including the maximum amount of time it can live in the bodies of survivors.

"We have to stop making intermittent recommendations and instead focus on the getting the right data to answer the question" of when Ebola is really gone from a community, said William Fischer, assistant professor of pulmonary and critical care medicine at the University of North Carolina, Chapel Hill.

So how will we know when the epidemic is truly over?

Right now, scientists don't have enough information to make that kind of declaration, Fischer said.

Conventional wisdom, gleaned from 40 years of Ebola outbreaks, says that the incubation period for Ebola is three weeks. That's based on research showing that 95% of people infected with Ebola will develop symptoms within 21 days. About 99% of people infected with Ebola will develop symptoms within 42 days, said Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston.

Those statistics have shaped the WHO's 42-day timeline for declaring a nation free of Ebola. The WHO starts the countdown to a country being declared Ebola-free on the day that the last known survivor's blood tests negative for Ebola virus.

But with an outbreak of this size — the largest in history — there could be nearly 300 people still incubating Ebola, even after 42 days, Hotez said. Those statistics could help explain why the Ebola has repeatedly appeared to disappear, then flare up unexpectedly, he said.

Conventional wisdom has also told scientists that Ebola survivors can be considered cured after two negative blood tests for the virus. Scientists believed these survivors could no longer "shed" the virus, making them unable to spread the disease.

Most of the time, people with no sign of Ebola in their blood are really cured.

But the massive size of the latest Ebola outbreak, which sickened nearly 29,000 people and killed more than 11,000, has given scientists a rare glimpse of exceptions to these rules.

A handful of cases — a Scottish nurse who relapsed months after recovery; a pregnant survivor who spread the virus after being declared cured; a male survivor who spread the disease through sex; an American doctor whose vision was impaired after his recovery  — now suggest the Ebola virus can "hide out" in certain places in the body, such as the testes, the brain, spinal cord and the fluid enclosed inside the eye, said Amesh Adalja, a senior associate at the Center for Health Security at the University of Pittsburgh Medical Center.

Given that evidence, using a 42-day benchmark to declare a country Ebola-free "represents a failure to learn from this epidemic," Fischer said.

Instead of starting the countdown on the day that the last survivor is declared cured, the WHO should start the clock when the last survivor stops "shedding" the Ebola virus, Fischer said.

There's just one problem with that proposal: No one knows how long that takes, Fischer said.

Scientists know that men can spread the Ebola virus through sex for at least nine months. But could the virus live in the testes for a year? More than a year? Doctors just don't know. But they need to find out, Fischer said.

Fischer said scientists should conduct careful studies of survivors' bodily fluids. About 17,000 people in West Africa have survived Ebola over the past two years, giving scientists an unparalleled opportunity to learn about the virus and potentially save lives in the next epidemic.

The National Institutes of Health began a 10-year clinical study of Ebola survivors in June. Researchers will study Ebola's long-term health effects on survivors; look for the presence of Ebola in semen; study whether survivors develop long-lasting immunity to Ebola; and observe whether Ebola survivors transmit the virus to close household contacts. Researchers have enrolled 1,800 participants so far and plan to enroll up to 7,500.

"The survivors deserve to be cared for and they deserve to be empowered with the ability to protect their loved ones," Fischer said. "We can accomplish this by taking the current infrastructure that was developed to respond to acute infection and pivoting it towards the care of survivors."