What Does It Mean for an Ebola Outbreak to End?

Nigeria’s most recent outbreak of Ebola is over, the nation’s government and World Health Organization (WHO) announced on Monday.

But — with fear of Ebola continuing to grip the world — what does that even mean? How does the WHO know that Nigeria is in the clear?

The answer, it turns out, is very specific: The WHO says a country can declare their outbreak to be over when it makes it through 42 days without a new case. That’s two incubation periods for the Ebola virus, so as long as 42 days have passed, during which the country had in place active surveillance and diagnostics but discovered no new cases, the WHO says it’s enough time to confidently say an outbreak is over. For health care workers to be considered “in the clear” they have to be monitored for 21 days after their last possible exposure to the virus, even if they were wearing full protective gear. Health care workers’ date of last contact is considered the day when the final patient with Ebola tests negative for the disease.

“Recent studies conducted in West Africa have demonstrated that 95% of confirmed cases have an incubation period in the range of 1 to 21 days; 98% have an incubation period that falls within the 1 to 42 day interval,” said WHO in a statement. “WHO is therefore confident that detection of no new cases, with active surveillance in place, throughout this 42-day period means that an Ebola outbreak is indeed over.”

MORE: Nigeria is Ebola-free: Here’s What They Did Right

This is not the first time WHO has declared Ebola outbreaks over using this particular standard — Senegal was declared Ebola-free on Oct. 17, and the strategy has proven effective in prior, unrelated, outbreaks.

In 1995, there was an Ebola outbreak in the country then called Zaire (today’s Democratic Republic of the Congo); it was declared clear on Aug. 25 of that year. The New York Times reported at the time:

The World Health Organization declared today that an outbreak in Zaire of the deadly Ebola virus was officially over after killing 244 of its 315 known victims.

The United Nations agency, which is based here, said that 42 days, the equivalent of two maximum incubation periods, had passed without any new cases reported. It said it was still not known where the Ebola virus existed between human epidemics, although samples from some 3,000 birds and mammals collected in the Kikwit area, the center of the outbreak, were now being analyzed.

It’s important to have definitive parameters for declaring outbreaks over because, as the current and former outbreaks have shown, oftentimes an outbreak will appear to be extinguished, only to reappear in full force a couple weeks later. This past April, Guinea’s health ministry thought the outbreak was slowing, which turned out to be false; in the 1995 outbreak, public health experts were also fooled. As TIME reported:

For a while last week it looked as though the outbreak might soon be brought under control. The plague police-medical teams dispatched by who in Geneva, the Centers for Disease Control and Prevention (CDC) in Atlanta and other public health groups-had set up an effective isolation ward at the main hospital in Kikwit, where the first case had been identified. Belgium’s Doctors Without Borders (Medecins Sans Frontieres, or MSF) rushed in loads of gloves, gowns, masks and other essential equipment to restore hygiene to filthy clinics. But when the strike forces, aided by local medical students, fanned out through the countryside around Kikwit, trying to follow the path of the fever, it became clear that the danger was far from past.

In an announcement made Monday morning, WHO called Nigeria a “spectacular success story,” citing proof that Ebola can be contained. “The story of how Nigeria ended what many believed to be potentially the most explosive Ebola outbreak imaginable is worth telling in detail,” WHO says in a statement.

To read more about how Nigeria contained their most recent outbreak of Ebola, check out our coverage, here.

Contact us at letters@time.com.