Officials in Guinea are working to contain the outbreak, which was identified as Ebola on Sunday. But the disease has had several weeks to spread, which could make it hard to stop quickly. The Democratic Republic of the Congo also has reported cases.

Ebola hasn’t been detected in the region since 2016, when it was responsible for more than 11,000 deaths. It was the worst outbreak of the disease to date—and Guinea was one of the most affected countries in that outbreak. The World Health Organization has alerted six countries in the region to be prepared for potential infections. It’s also helping provide Guinea with the Ebola vaccine, which has slowed outbreaks in the Congo.

Ebola, historically, has killed 90% of the people who have become infected with it. But vaccines and experimental treatments have lowered death rates to roughly 66%, according to the WHO.

“It’s a huge concern to see the resurgence of Ebola in Guinea, a country which has already suffered so much from the disease,” said Dr. Matshidiso Moeti, the WHO regional director for Africa, in a statement. “However, banking on the expertise and experience built during the previous outbreak, health teams in Guinea are on the move to quickly trace the path of the virus and curb further infections.”

Guinea has announced 10 suspected cases of Ebola and five deaths with this latest outbreak. The Congo has confirmed four new cases this month—a particularly worrisome sign, as the last outbreak in that country, which killed 2,299 people, was declared over in June.

The Ebola worries come as health care officials are also keeping a close watch on the so-called South African variant strain of COVID-19, which is about 50% more contagious and has quickly spread throughout the country and the world. In the U.S., cases have been reported in South Carolina and Maryland.