A rare Ebola strain is spreading. Here's why it's so hard to contain.

The World Health Organization has declared a Public Health Emergency of International Concern following an Ebola outbreak in the Democratic Republic of the Congo (DRC) and in neighboring Uganda. The declaration is the second-noisiest alarm the agency can ring on an outbreak, and a signal to other member states to activate their national response and preparedness systems. WHO Director-General Tedros Adhanom Ghebreyesus noted in remarks to press that he “determined that the situation was not a pandemic emergency,” the agency’s new and highest classification for outbreaks.

As of May 20, officials say the outbreak has led to 139 deaths and nearly 600 suspected cases. Fifty-one cases have been confirmed in towns in the remote, densely forested northeastern Ituri province, and Uganda has confirmed two cases.

(What is the Ebola virus, and can it be stopped?)

For Nahid Bhadelia, an infectious disease doctor and founding director of Boston University’s Center on Emerging Infectious Diseases, the initial details took her back to 2014’s massive West African Ebola epidemic. 

“The delay of months for identification, the presence in multiple urban areas before detection, cases in many different geographically disparate places,” she said—all these were parallels to the present and indicated an outbreak that had simmered undetected for some time before boiling over.

But how much risk does the Ebola outbreak pose to the rest of the world? Here’s what you need to know about Ebola virus and how it spreads. 

How does Ebola spread, and why is it so dangerous?

Ebola viruses belong to the Filovirus family, which also includes Marburg viruses. Four species of Ebola viruses cause disease in humans: Zaire is most common, while the one responsible for this outbreak, Bundibugyo, is rare. That means it’s harder to diagnose and, unlike the Zaire virus, there are no vaccines or treatments for it. 

Like other Ebola viruses, Bundibugyo causes flulike symptoms—fever, muscle aches, and fatigue—that often progress to severe diarrhea, vomiting, and bleeding. Symptoms vary enough from person to person that infections can often be mistaken for malaria or typhoid if diagnostic testing isn’t done. It’s a dangerous disease: Between one-third and half of people who get infected with the virus die as a result.

(Is malaria making a comeback in the U.S.?)

Although it’s not clear who was the first person infected in this outbreak, its explosive spread is thought to have started with a death on May 5, according to Anne Ancia, a World Health Organization representative in the DRC. Speaking at a press conference on May 19, she said the body of a person who died in Bunia, the capital of the Ituri province, was transported to a smaller town, then moved from their original coffin because “the family decided that the coffin was not worth the person.” 

Ebola outbreaks usually start when someone becomes infected during contact with an Ebola-infected animal. They spread when people have direct contact with the body fluids of others who are sick or have died from the infection, including urine, saliva, sweat, feces, breast milk, amniotic fluid, and semen. Importantly—and in contrast to COVID-19, flu, and many other infections—infected people without symptoms are unlikely to transmit Ebola to others.

In the parts of sub-Saharan Africa where Ebola is most common, the infection often spreads when family members touch the body of a loved one during burial preparations, then have contact with others attending the funeral; something similar likely lit this outbreak’s spark, Ancia said.

Surviving Ebola largely depends on early diagnosis and getting the kind of care people receive in intensive care units—like intravenous rehydration and medicines to maintain blood supply to vital organs. For that reason, deaths due to Ebola infection are highest in places where access to health care is lowest.

Why is it so hard to stop an Ebola outbreak?

Stopping an Ebola outbreak requires health authorities to do many things all at once: They have to find ways to isolate sick people during their treatment, while also protecting the healthcare workers who provide their care; trace all contacts of sick people and monitor them for weeks; and ensure safe and dignified burials for those who’ve died. 

These efforts are sometimes complicated by intense distrust of unfamiliar medical responders, especially in remote regions traumatized by conflict. Engaging affected communities through trusted messengers can make or break a response. 

Several factors make Ebola outbreaks especially difficult to contain in this region of the DRC, which has seen several Ebola outbreaks before. Mining outfits dotting the largely rural region bring humans in contact with the wildlife reservoirs for Ebola and related viruses, and their camps often house people in close quarters with limited hygiene measures and healthcare access. As a result, infections in these regions tend to spread widely before they even catch the attention of health authorities. 

(What’s keeping scientists from vanquishing Ebola?)

Ongoing armed conflict in the area also creates conditions that favor viral spread locally and beyond the DRC’s borders, especially when international observers and funding are abruptly cut off as they have been recently, says Bhadelia. Less funding to diagnose and treat malaria and other Ebola mimics means it’s harder to find an Ebola signal in the noise of many other flu-like illnesses. Furthermore, borders in Central and East Africa are porous and the capacity to identify, isolate, and treat Ebola is scarce, fueling fears that the virus will spread beyond Uganda into other neighboring countries. 

In a press conference May 20, WHO Senior Advisor Vasee Moorthy said Erevbo vaccine, which is used to protect humans against the Zaire strain of Ebola, “needs to be prioritized as the most promising Bundibugyo candidate vaccine.” The vaccine has shown some activity against Bundibugyo in animal studies, but would need to be trialed in humans prior to use. Another possible candidate is a formulation based on the same ChAdOx1-S platform used to develop the AstraZeneca COVID-19 vaccine, said Moorthy, although it has not even been trialed against Bundibugyo in animals. It will be months before either candidate could be deployed as a countermeasure in the current outbreak.

How much of a threat does this outbreak pose outside of the region?

Suffering and risk are currently intense in the areas where the disease has been detected, says Bhadelia. At the heart of Ebola outbreaks, she said, "Healthcare workers are making these decisions about, ‘Oh my god, do I see this patient?’ and 'Do I reuse PPE?'”

The way Ebola is transmitted—by close contact with body fluids, not through respiratory routes—and the broad availability of tools to diagnose and respond to the disease in high-resource countries means the infection is not a threat to the international community, says Bhadelia. If people can only spread infection while they’re symptomatic, and only to people with whom they have close contact, massive airport outbreaks are unlikely. And if infected travelers fall ill after arriving in a high-resource country, medical providers are likely to have good enough infrastructure to catch the infection before it spreads widely—much as they did, with some lessons learned, during the 2014-2015 epidemic.

In the United States in particular, infectious threat response systems like the National Special Pathogen System have created “a ton of capacity” to treat infected patients and protect health care workers, she said.

There’s a possibility sporadic cases could be imported into the U.S., but Bhadelia is not concerned that would lead to a stateside pandemic. What worries her more is the possibility that such cases would distract our public health resources to the point they’d miss signals of an actual pandemic if it unfolded at the same time.

There’s no need to panic about Ebola, she says, but “I think we need to panic about the state of the global health situation.”