Sierra Leone has officially been declared free of Ebola by WHO

With 42 days having passed since the last negative blood test from an Ebola patient in Sierra Leone, the World Health Organization has declared the deadly virus is no longer being transmitted in the country. Ebola killed more than 3,500 people in Sierra Leone's outbreak that began in May of 2014.

Marking the occasion Saturday, Dr. Anders Nordström of the WHO says that in Sierra Leone, "8,704 people were infected and 3,589 have died, 221 of them healthcare workers, all of whom we remember on this day.".

The 42-day mark is significant because it represents two cycles of Ebola's incubation period. Health officials will now monitor the situation in Sierra Leone for 90 days, looking for any potential resurgence of the virus.

While celebrating the milestone, infectious disease expert Dr. Nahid Bedalia, who worked in Sierra Leone during the epidemic, says that for people who survived Ebola, the WHO declaration "does not mean the end of the impact of this disease on their lives."

Writing for for NPR's Goats and Soda blog, Bedalia says, "The last month has significantly altered our understanding of how Ebola virus interacts with the human body," citing new research showing that the virus can manifest in other forms such as meningitis, and that it can "survive in the sperm of a quarter of male Ebola survivors up to nine months after recovery."

And as NPR's Jason Beaubien reports, even after the Ebola outbreak was declared over in neighboring Liberia last spring, a few more cases arose later.

"Guinea is the only country from this outbreak where the virus continues to spread," Jason says. "Guinea reported six cases last month."