When news broke this week that the Democratic Republic of the Congo is facing yet another Ebola outbreak, many public health experts were not surprised. The vast central African country has dealt with more outbreaks of this often-fatal hemorrhagic disease than any other nation. Yet exactly why the DRC is hit so often remains an unanswered question.

The DRC Ministry of Health announced the latest outbreak this week after laboratory testing confirmed two cases had occurred in the northwestern part of the country, near the its border with the similarly named Republic of the Congo. This is DRC’s ninth Ebola outbreak since scientists first identified the disease.

Most of the country’s outbreaks burn out quickly because they occur in relatively remote areas—but each has had a high fatality rate. The world’s first-known Ebola cases occurred in the DRC in 1976 (when the country was called Zaire), killing 280 of the 318 people known to have been infected. Last year eight cases were reported, and half of the infected people died.

The disease spreads among humans through direct contact with the bodily fluids of a person who is sick with or has died from Ebola. It can be transmitted via sexual contact. It can also spread to people who encounter the bodily fluids or viscera of infected primates or bats, such as when someone prepares these animals for food. Most outbreaks have originated in Africa or from lab accidents elsewhere, and have remained small—but people traveling from sites of a 2014 outbreak that roiled west Africa led to isolated cases in locations including the U.K., Italy, Spain and the U.S.

Ebola experts have various suspicions about why the DRC remains so vulnerable. Most theories involve the country’s large forested areas, and the possibility that infected fruit bats—widely believed to be the primary reservoir animal for the disease—are common in the affected areas. “If you live or work in a forest where bats roost, you may have the bad luck to be in touch with bat guano from an infected animal. Or perhaps you encountered saliva or guano on a piece of fruit which an infected fruit bat was involved with,” says Daniel Bausch, a veteran Ebola responder and director of the U.K. Public Health Rapid Support Team. In certain areas of the DRC—including where Ebola has been reported recently—the disease can now be considered endemic in reservoir animals, he says.

During the massive Ebola crisis that gripped west Africa from 2014 to 2016, researchers were told about a bat-filled roosting tree in Guinea they suspected may have been ground zero for that outbreak. Rounding up some of those bats could have given scientists a chance to test the animals and confirm them as a main reservoir for the disease. But the tree burned down and the bats were reduced to char, destroying the opportunity to search for much evidence—although researchers were able to use trace DNA fragments to pinpoint that they were Mops condylurus, an insect-eating species common across central and west Africa.

Whether experts will be able to find infected bats during the latest DRC outbreak remains unknown, partly because there may have been undetected cases of the disease dating back months; any culprit bat colony may have moved on by now. “If the outbreak started last month, then yes, it would be interesting to sample the bats. But if it started in December, then no, the best thing to do may be come to back next December” (when the same type of bats may return to the area), says Pierre Rollin, an Ebola expert at the U.S. Centers for Disease Control and Prevention who has responded to most known Ebola outbreaks.

Peter Piot, director of the London School of Hygiene and Tropical Medicine, who was part of the team that originally discovered the virus in 1976, also believes people’s interactions with infected bats may be behind the repeated outbreaks there. “Due to its huge swathes of forest, the DRC is a reservoir for the virus, making the country particularly susceptible to outbreaks of Ebola,” he says. “Occasionally people living in these rural areas will come into contact with infected animals, and the transmission cycle begins.” Some experts also suspect deforestation could be a factor, bringing infected animals and people together in the area when they may cut down trees or butcher infected bats for food. DRC is about the size of Greenland—almost a million square miles—and much of it has long been forested, notes Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

The extent to which individual countries can detect, confirm and report the disease may also affect scientists’ understanding of why it occurs where it does. Whether or not the DRC is truly seeing the disease much more than some of its neighbors is debatable, Rollin says. “I think there could be surveillance bias—it’s difficult to say,” he says, noting the neighboring Republic of the Congo had multiple Ebola outbreaks in the early 2000s, although none have been reported since 2003. “The reservoir [animal] is in DRC, Gabon and the Republic of [the] Congo,” he says. So what’s different? The DRC has more robust Ebola surveillance and lab testing capacity in place, he says, and that may be a factor.

The U.K.’s rapid response team and the CDC are both standing by to help respond to the latest outbreak, and are consulting with the World Health Organization and the DRC’s Ministry of Health. But the British and U.S. organizations are not yet part of the formal response on the ground.  The formal response team—WHO, Doctors Without Borders and the DRC health ministry—have just arrived in the area, Rollin says, so there are still many unknowns.

In the latest DRC outbreak, “it is concerning that of the 21 reported cases so far, 17 are fatal. That makes me think this is the tip of the iceberg. But how large the iceberg is right now, it’s too early to say. It is appropriate to be taking this seriously and mounting a firm response,” Bausch says. “The village at the heart of the current outbreak is remote by land but it is on a major river with access to major population centers via the water route. And there is always concern about connections between populations.”