Ebola is one of those scourges where the mere mention of its name strikes fear: the virus, which kills about half of those it infects and gets passed on through body fluids, is notoriously hard to contain.

That means that the best, perhaps only, way to contain an outbreak like the one currently ravaging the Democratic Republic of Congo is by obsessively tracking infected individuals—monitoring their social circles and their movements, and limiting their exposure to other people for weeks at a time. But containment is proving so difficult in DRC that last week, Robert Redfield, director of the Centers for Disease Control and Prevention, suggested an alarming possibility. The current Ebola epidemic could be beyond control, he said, and may—for the first time since the deadly virus was first identified in 1976—become persistently entrenched in the population.

The 329 confirmed and probable cases of Ebola infection reported so far have made it the largest outbreak in the nation’s history, with no signs yet of slowing down. Militia groups clashing in DRC’s North Kivu Province, the epicenter of the outbreak, have scrambled health workers’ attempts to trace the movements of people exposed to the virus. A massive effort to vaccinate more than 25,000 of the highest-risk people has slowed transmission rates but not yet stemmed the tide. Between October 31 and November 6, 29 new cases were reported in DRC, including three health workers.

Now neighboring Uganda is bracing for the virus to cross the 545-mile boundary it shares with DRC. The border is porous and heavily trafficked, with large numbers of local farmers, merchants, traders, and refugees constantly moving through the area. A checkpoint in the region receives 5,000 people on an average day, with the busiest ones swelling to 20,000 twice a week on market days.

On Wednesday, the country began immunizing frontline health workers with an experimental vaccine that produced good results in a previous outbreak. The Ugandan Health Ministry said it has 2,100 doses of the vaccine available for doctors and nurses working in five border districts. At hospitals in these districts, four special Ebola treatment units have also been constructed, with staff on standby to manage any suspected cases. “The risk of cross-border transmission was assessed to be very high at a national level,” said Ugandan Health Minister Jane Ruth Aceng at a press conference last week. “Hence the need to protect our health workers.”

Since the start of the outbreak in the DRC, anyone crossing into Uganda has been subjected to health screenings at official checkpoints—a series of questions and no-contact infrared thermometers aimed at the side of the head that read out body temperatures like a highway patrolman’s radar gun. Fever is one of the first red flags for an Ebola infection. The process isn’t foolproof; symptoms can take up to three weeks to appear, and lots of other tropical diseases in that part of Africa can also cause soaring temperatures.

The abundance of caution arises from the unstable situation in the DRC. Ebola has never before broken out in a war zone, so in many ways the current situation is unique and unprecedented. But as larger changes have swept across the African continent—ballooning populations, billions of dollars of Chinese infrastructure investments, the increase of urban-wildland interfaces—some infectious disease doctors see a lasting change to the shape of Ebola outbreaks. "It's a cruel irony that better roads and improved connectivity of people also make it easier for the disease to travel, particularly when the public health systems are still lagging behind, “ says Nahid Bhadelia, medical director of Boston Medical Center's Special Pathogens Unit, who worked on the front lines of the 2014 outbreak in Sierra Leone.