The fastest-growing Ebola outbreak on record began in February, months before it was officially declared in mid-May, the World Health Organization said Monday.

The WHO regional director for Africa, Dr. Mohamed Yakub Janabi, told a news conference that genetic sequencing indicated the outbreak’s true start, with some early cases wrongly attributed to malaria, typhoid or a more common type of Ebola.

“This is a four months’ delay,” he said. “So we are chasing the virus. The virus is ahead of us.”

Confirmed cases in the outbreak have reached 4,200, including more than 1,900 deaths, according to the latest government figures.

Declared on May 15, it is unlike most previous Ebola outbreaks because the rare Bundibugyo virus responsible has no approved vaccines or treatments.

Janabi said many patients were dying because they reached care too late.

“We are seeing a lot of patients in Bunia dying,” he said, explaining that villages outside the town lack treatment services, forcing people to travel to Bunia. “Unfortunately, they come late.”

The outbreak is unfolding under some of the most challenging conditions imaginable, with strikes by some unpaid health workers, threats from rebel groups, anger from long-traumatized communities and misinformation asserting that Ebola isn’t real.

Health teams travel remote, unpaved roads and have reported shortages of protective gear.

Steve Ahuka, the national Ebola incident manager, said the single biggest challenge was reaching communities.

“We need to raise awareness, we need to explain more to our fellow citizens, because a population that has understood the disease… will itself report cases,” he said.

This outbreak has killed people faster than any other, including the 2014-2016 West Africa outbreak, the worst on record, which had more than 11,000 deaths among at least 28,000 cases but took about eight months to reach 1,000 deaths.