Guinea, one of the countries hit by the worst outbreak of Ebola, has been declared free of any cases of the deadly virus.

Because Guinea hasn't seen any new infections for 42 days, the amount of time in two incubation cycles for the disease, the WHO declared it free of Ebola on Tuesday. The last known patient in Guinea was a 3-week-old girl who tested negative for the virus twice in November, The New York Times reported.

"It's cause for celebration, because people are day to day living in very tough conditions," said Susan Michaels-Strasser, a professor at Columbia University's Mailman School of Public Health and a nurse who worked in Sierra Leone during the outbreak. "Everyone should be embarrassed that it got to such a level. The warning signs were there."

There will be a ceremony on Wednesday in which government officials, NGOs and foreign representatives will mark the declaration.

My colleagues and I at the World Bank Group congratulate the government and people of Guinea on reaching this important milestone. They have showed remarkable determination fighting Ebola since the first case emerged in rural Guinea two years ago," said Jim Yong Kim , president of the World Bank, in a statement. "Still, we must remain vigilant to stay at zero cases, and continue to support Guinea as it contends with the enormous human and economic costs of Ebola."

The worst outbreak of Ebola started with a patient in Guinea two years ago and quickly spread across porous borders into neighboring countries of West Africa and as far away as the United States. The World Health Organization reports that the crisis killed more than 11,000 people, mainly in Guinea, Liberia and Sierra Leone.

The first fatality from the current outbreak was traced to a 2-year-old boy in Guinea in December 2013, though the WHO didn't identify the disease until March 2014. Transmitted through contact with the bodily fluids of an infected person, Ebola killed more than 2,500 people in Guinea.

Controlling the outbreak strained the resources of countries like Guinea, which had a weak public health system. The epidemic originated in a rural area, where the government's presence is even less visible. The poorly understood disease, which causes violent diarrhea and vomiting, also created widespread panic.

An experimental drug, Zmapp, was first used on an American medical missionary infected, and later was used on patients in Guinea and other countries. But its effectiveness hasn't been determined and medical experts have said that the outbreak was finally brought under control by educating people about how it was passed from one person to another, according to Dr. Joel Selanikio, a pediatrics professor at Georgetown University who treated patients in Sierra Leone.

"The key was to separate infected people from contact with the community," said Selanikio. "It's not voodoo magic. It's a transmissible disease."

And it's one that required a tremendous outlay of resources. The World Bank alone provided $250 million in financing to Guinea to combat the epidemic.

Hysteria gripped the United States after the Centers for Disease Control and Prevention confirmed a case of Ebola in Dallas, but coverage soon slipped from newspaper front pages and cable news broadcasts.

But Ebola persisted in places like Guinea, and its full impact is still being assessed. The United Nations Developmental Group estimated that the region could lose $3.9 billion per year through 2017 from things like loss of trade and foreign investment.

The disease had other consequences. Because of fears about Ebola , approximately 74,000 cases of malaria went untreated in Guinea in 2014, according to a report in the medical journal The Lancet.

Being declared free of Ebola is no guarantee that a new case might not suddenly appear. Liberia has twice been declared clear of human cases, most recently in September, but last month, a 15-year-old boy there died from the virus. Sierra Leone has been rid of Ebola since early November.

CORRECTION: An earlier version of this article misstated in which country Susan Michaels-Strasser worked during the epidemic. She made three trips to Sierra Leone.