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Ebola’s back, just hours after West Africa officially hit zero.

A woman who died in Sierra Leone was diagnosed with Ebola Thursday evening, the same day that the World Health Organization said it had been 42 days since the last known Ebola case in the region. WHO confirmed the diagnosis Friday morning.

The case shows that while a devastating, two-year-long epidemic of the deadly virus may be over, that doesn’t mean Ebola’s gone from West Africa.

"We are now at a critical period in the Ebola epidemic as we move from managing cases and patients to managing the residual risk of new infections,” said Dr. Bruce Aylward, special representative for Ebola at the WHO.

“We still anticipate more flare-ups and must be prepared for them.”

Ebola came seemingly out of nowhere to West Africa around two years ago. It had never been seen there before, although it caused much smaller outbreaks in central and eastern Africa.

“We still anticipate more flare-ups and must be prepared for them.”

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It spread quickly from Guinea to Sierra Leone and Liberia across porous borders in a densely populated area where resources had been devastated by wars. Ebola has infected at least 28,000 people – a number that officials fear is almost certainly an undercount – and killed more than 11,000. That’s probably an undercount, too.

Many of the victims died without ever having been diagnosed.

No one’s sure where the virus came from in the first place, although bats are the No. 1 suspect. Other animals can carry it, also, and people in the region frequently hunt food in the forests.

But the biggest threat to people is other people. Ebola is not highly infectious unless a patient is very ill, but then those caring for the victim are extremely vulnerable because the infection causes severe diarrhea, vomiting and sometimes hemorrhaging.

And it’s now clear that a few people may remain infectious for months after they’ve seemingly recovered.

“Guinea, Liberia and Sierra Leone remain at high risk of additional small outbreaks of Ebola in the coming months due to the virus persisting in survivors after recovery,” WHO said.

World health experts are on high alert for now. Just this week yet another report came out saying that the world isn’t prepared for a pandemic of infectious disease and that the consequences could run into the billions of dollars, let alone lives.

"The bigger problem is that leaders at all levels have not been giving these threats anything close to the priority they demand. Ebola and other outbreaks revealed gaping holes in preparedness, serious weaknesses in response, and a range of failures of global and local leadership," the National Academy of Medicine says in its report.

“The world must prepare for future pandemics now,” World Bank Group President Jim Yong Kim said in a statement.

“The world must prepare for future pandemics now."

“Expert reviews of the Ebola response led by the United Nations, the Harvard Global Health Institute and the London School of Hygiene & Tropical Medicine, the National Academy of Medicine, and others offer the opportunity to build a smarter, swifter global pandemic response system," Kim added.

"As part of this effort, the World Bank Group, World Health Organization, and government and private sector partners are developing a Pandemic Emergency Financing Facility to provide swift and flexible financing to help stop a severe outbreak from becoming a more deadly and costly pandemic, thereby saving lives and protecting economies.”