(CNN) The World Health Organization has declared Guinea to be free of Ebola.
It's a milestone, as the Ebola outbreak that ravaged West Africa began in Guinea in December 2013 and spread to Liberia and Sierra Leone. From there, the outbreak traveled to seven other countries.
Liberia and Sierra Leone were earlier declared Ebola-free.
The declaration was made because 42 days have passed since the last person confirmed to have Ebola tested negative for the second time, a WHO statement said.
But the county now enters a 90-day period of heightened surveillance to make sure that any new cases are identified quickly, the WHO said.
The statement included praise for the government of Guinea and its people, which the WHO said had "shown extraordinary leadership in fighting the epidemic."
However, the global health organization said that challenges remain -- among them, that the virus may persist in the semen of some survivors for up to a year.
The disease is passed through direct contact with the bodily fluids of an infected person.
"The coming months will be absolutely critical," said Dr. Bruce Aylward, the head of Ebola response for the WHO.
Researchers from The New England Journal of Medicine say they think the first person to become infected with the disease was Emile Ouamouno, a 2-year-old boy who lived in Gueckedou, a town in southern Guinea.
The total number of people infected with Ebola eventually totaled more than 28,000, according to the World Health Organization. And the number of people who died from the disease reached more than 11,200.
Among those who died was Emile, patient zero.