The Ebola outbreak spreading through the Democratic Republic of Congo could become the deadliest in history, aid groups have warned, outpacing the international response.

As of Tuesday, the number of confirmed cases was at 321 in Congo, with 166 more suspected cases, according to the World Health Organization, and nine confirmed cases in neighboring Uganda. The number of confirmed deaths from the virus was 49 as of Tuesday.

Congo has faced more than a dozen Ebola outbreaks in recent decades, but this one, the 17th, has tested the country’s expertise. At its center, health workers and aid groups are racing to fight the virus, but little infrastructure is in place to slow its spread.

The type of Ebola virus behind this outbreak, known as Bundibugyo, is rare, with no targeted vaccines or treatment. Early surveillance and testing failed to identify it, delaying the response, and fighting in the eastern Congo has displaced thousands of people, making it even harder to trace.