The rainy season in West Africa is compounding difficulties in getting supplies delivered and new treatment centers built as donors rush to isolate people infected with the deadly Ebola virus and stop its rapid spread, U.S. officials said.

Nancy Powell, newly appointed as the U.S. State Department’s envoy to coordinate its Ebola response, said the priority is to isolate as many people as quickly as possible. But that faces significant logistical hurdles.

“Infrastructure challenges in the rainy season is one of the biggest difficulties. And you add the rain and getting materials out of the capital and it is very difficult,” Powell said in a news briefing last week.

The July to September rainy season is coming toward its end, but October is known for heavy thunderstorms that can drench the region and turn roads to mud.

Eric Talbert, executive director of Emergency USA, which has opened a 22-bed Ebola treatment center in Goderich, outside the capital of Freetown in Sierra Leone, said the downpours complicate getting supplies along unpaved roads.

“We are talking about dirt roads that are single track. The rains wash them out. … Rains are not only going to escalate the logistical difficulties, but Ebola will spread if we cannot reach people and treat them,” Talbert told the Thomson Reuters Foundation.

Powell said delivering supplies and trained staff quickly is essential in controlling the spread of the epidemic.

“This is the key way to bend the curve, isolate people and track people with whom they have had contact,” Powell said. “But the numbers make it difficult to do, and the bigger the case load, the harder it gets to do.”

More than 6,000 people in West Africa are estimated to have contracted the highly contagious Ebola virus, which causes fever, vomiting, bleeding and has a death rate over 50 percent. But infections probably are far higher, especially in rural areas, and researchers estimate the epidemic could reach 20,000 people by November.

The U.S. military is helping ferry medical supplies into Liberia and within the next month plans to construct 17 new Ebola treatment units. Britain is building centers in Sierra Leone.

International donors in parallel are distributing supplies to the far-flung rural communities where they want to isolate infected patients if it is too dangerous to transport them to a hospital.

This new focus on community care in parallel with building Ebola treatment centers requires getting supplies into rural areas on roads that can become impassable after heavy downpours.

However, modelling by the African Center of Meteorological Applications for Development points to relief ahead. It forecasts rainfall well below average through December for Guinea, Sierra Leone, Liberia and Senegal.