DAKAR, Senegal — Guinea has been declared free from transmission of Ebola, the World Health Organization said Tuesday, marking a milestone for the West African country where the original Ebola chain of transmission began two years ago leading to the largest epidemic in history.

The end of #Ebola transmission in #Guinea marks an important milestone https://t.co/JOXfhqzzTU pic.twitter.com/wuA1gxvzDV

— WHO (@WHO) December 29, 2015

The U.N. organization is holding a ceremony Tuesday in Conakry, the capital, to mark the step forward. More than 2,500 people have died in Guinea from Ebola. The deadly virus has killed more than 11,300 people worldwide, mostly in Liberia, Sierra Leone and Guinea.

Guinea will now enter a 90-day period of heightened surveillance. Dr. Mohamed Belhocine, World Health Organization representative in Guinea, said the organization and partners will continue to support Guinea through this period and “in its early efforts to restart and strengthen essential health services throughout 2016.”

The World Health Organization declares that Ebola disease transmission has ended when the country goes through two incubation periods — 21 days each — without a new case emerging.

Guinea was the last to struggle to stamp out the deadly disease, until Liberia saw a new case in November. Sierra Leone was declared free from transmissions Nov. 7. Liberia was declared Ebola-free twice, but has entered a third countdown after the new cases emerged.

“This is the first time that all three countries – Guinea, Liberia and Sierra Leone – have stopped the original chains of transmission that were responsible for starting this devastating outbreak two years ago,” said Dr. Matshidiso Moeti, WHO Regional Director for Africa.

Experts warn that vigilance is needed. The deadly virus has been found to persist in the semen of some male survivors for as long as 9 months to a year. More than 15,000 survivors also face health issues linked to the virus.

“The coming months will be absolutely critical,” said Dr. Bruce Aylward, Special Representative of the World Health Organization Director-General for the Ebola Response. “This is the period when the countries need to be sure that they are fully prepared to prevent, detect and respond to any new cases.”

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