An attack on a hospital at the epicenter of an Ebola outbreak in the Democratic Republic of the Congo on Friday has left at least one health worker dead and others injured, according to the World Health Organization’s director general Dr. Tedros Adhanom Ghebreyesus.

“We are outraged by the attack,” Dr. Tedros, as he’s known, said on Twitter.

The attack occurred at a hospital in Butembo, one of the hotspots in an outbreak that’s been ongoing since last August. At least 1,200 people have contracted the virus and 830 have died.

It’s the first known Ebola epidemic in a war zone (DRC’s eastern North Kivu and Ituri provinces) — and now the second-largest in history. But several attacks on aid workers have set the response back.

In March, an Ebola treatment center that shut down in response to an arson attack weeks earlier was again stormed by armed assailants, leaving one police officer dead, health workers injured, and patients fearful for their safety, according to the Associated Press.

Another setback came on February 24, in Katwa. Late at night, assailants threw stones at a Doctors Without Borders (MSF) building and then proceeded to set it on fire, forcing workers to evacuate and to move patients to other hospitals.

Days later, on February 27, arsonists set MSF’s Butembo operation on fire. The facility was rapidly rehabilitated and reopened on March 2, operated now by DRC’s Ministry of Health, WHO, and UNICEF. But a week later, on March 9, it was attacked again.

The latest attack, however, is the first time a WHO health worker responding to this Ebola outbreak has been killed.

“Attacks such as this one are becoming increasingly common and with each attack on a hospital or centre tackling Ebola, the disease gains traction,” said Bob Kitchen, the senior vice president for emergencies at the International Red Cross. “This month we are on track to see the highest rate of transmission in a single month, following March’s already record-breaking numbers.”

The interruptions in the Ebola response are disturbing for two reasons. First, they are hugely tragic: Health workers trying to stop the spread of a disease are being targeted at work. Second, they mean fewer workers will be available to identify, isolate, and treat Ebola patients. And in an Ebola response, when people with the virus remain in the community, they can spread it to others.

But even worse, the attacks suggest that at least some community members are resisting the humanitarian efforts to stop the outbreak. They may be a minority, but they pose a significant threat to the Ebola response. Instead of cooperating with aid workers, they’re burning down the medical units set up to protect them. That’s a major setback for DRC and its Ebola responders.

The success of this Ebola response hinges on trust. The attacks suggest there’s a trust problem.

Stamping out an Ebola outbreak requires a very high level of community buy-in. The virus is spread through direct contact with the bodily fluids — such as vomit, urine, or blood — of someone who is already sick and has symptoms. The sicker people get and the closer to death, the more contagious they become. (That’s why caring for the very ill and attending funerals is especially dangerous.)

Because we have no cure for Ebola, health workers rely on traditional public health measures: finding, treating, and isolating the sick, and breaking the chains of transmission so the virus stops spreading.

They mount vigorous public health awareness campaigns to remind people to wash their hands; that touching and kissing friends and neighbors is a potential health risk; and that burial practices need to be modified. (Again, funerals can act as Ebola super-spreaders since people who die of the virus are extremely contagious, and families prepare, touch, and kiss corpses as part of traditional funerary rituals.)

Responders also employ a strategy called “contact tracing”: finding all the contacts of people who are sick and following up with them for 21 days — the period during which Ebola incubates.

Every one of these measures can only succeed if community members trust and cooperate with Ebola responders.

They need to let aid workers into their home to test for the virus and agree to be followed up with for 21 days. They need to accept the painful reality of sending family members with the virus to treatment centers for isolation so they can’t infect others around them.

They need to follow basic public health measures such as hand washing and safe burial practices, which can mean putting aside centuries of family tradition and personal beliefs in order to stop the spread of the virus.

In this case, responders also have an effective vaccine at their disposal — and to date, more than 86,000 people have been vaccinated, including in Katwa and Butembo.

But that doesn’t necessarily mean people are cooperating with the other parts of the Ebola response. A recent Lancet survey of locals’ attitudes toward the Ebola response in DRC suggested that people’s willingness to get the Ebola vaccine was really high (82 percent said they’d take the vaccine for their family members) — but many respondents said they would not send their family to treatment centers (17 percent) and would actively hide family members with Ebola from the authorities (17 percent).

“To overcome [an Ebola] outbreak, we need to gain the hearts of the population,” Michel Yao, the WHO’s incident manager in Butembo, told Vox in March. “We need to have them fully on board.” Yao believes community resistance has been improving overall, but the latest attack at the hospital suggest there may be more trouble ahead.

Clarification: We’ve updated the story to add the figures from the Lancet study clarifying DRC attitudes toward the Ebola response.