This July, the World Health Organization declared that an outbreak of Ebola in the provinces of Ituri and North-Kivu, in the eastern Democratic Republic of the Congo, was a “public health emergency of international concern.” This particular strain of the virus, which first appeared in the region in 2018 and hasn’t been given a formal name—I’ll call it Kivu Ebola—is a variant of a species known as the Zaire Ebola virus. As of last Saturday, 2,753 cases of Kivu Ebola have been reported, with 1,843 deaths. There appear to be many undiscovered cases in the region, too. Ella Watson-Stryker, a social scientist with Doctors Without Borders, who has been studying the outbreak, said that around half of all Ebola patients admitted to treatment centers in eastern Congo aren’t part of any known chain of transmission. In other words, the infected person has caught Ebola from somebody whom disease investigators haven’t yet identified. “A lot of transmission is not being seen, but nobody knows the exact amount,” Watson-Stryker told me.

Ebola virus is a microscopic parasite that replicates inside the cells of a host. The outbreak in eastern Congo began more than a year ago, in or near a town called Mangina, when a few particles of Ebola virus apparently moved out of some wild creature, Ebola’s natural host—in this case, probably a bat—and entered the bloodstream of an as yet unidentified person. From that person, the virus began spreading through the local population. Ebola can overwhelm the human immune system in a matter of days. Symptoms typically include vomiting, diarrhea, coughing, rash, dementia, hemorrhages, and hiccups. Death occurs like the slamming of a door, when the patient abruptly goes into shock.

The Kivu Ebola outbreak area is in a conflict zone, beset by armed militias and ethnic violence. Local people often don’t trust the international medical organizations that run the Ebola treatment centers. There have been at least a hundred and ninety-four attacks on local health workers, seven of whom have been killed. Watson-Stryker, the researcher, said that social media complicates containment and treatment efforts. Conspiracy theories about medical workers and false information about how the virus is spread are ricocheting around popular platforms like WhatsApp. “The problem is the post-factual reality that exists in social media,” she said.

An effective experimental vaccine for Ebola exists, and more than a hundred and seventy-five thousand people have received it. Even so, the virus is finding new victims and extending its geographic range. Three cases of Ebola recently appeared in Uganda, and there have now been four cases in the Congolese city of Goma, which has roughly two million residents and is situated on the border with Rwanda. The W.H.O. recently estimated that more than two hundred million dollars in emergency funding would be needed to bring the virus under control. That money hasn’t been raised yet.

An Ebola particle is a very small, filament-shaped object, made of six different structural proteins. Ebola’s genetic code, or genome, is contained in a strand of ribonucleic acid, or RNA, that is coiled tightly in the core of the particle. The genome, which has some nineteen thousand letters in it, holds the master designs of Ebola’s proteins.

RNA viruses—which range from Ebola to measles and influenza— tend to produce errors, or mutations, in their code when they copy themselves. Most mutations are either bad for the virus or have no effect on it. Every now and then, however, a virus gets a mutation that benefits it. In fact, the production of errors during copying plays an important role in the long-term survival of viruses. As time goes by and the virus makes inaccurate copies of itself, slightly different varieties of the virus arise. The different varieties are called lineages. They can be imagined as moths of the same species whose wings are slightly different colors. Some wing colors help a moth camouflage itself more effectively, be eaten less often by predators, and survive longer than moths of other colors. Those types of moths go on to reproduce successfully, while moths of other colors eventually die out, until the population of moths has changed color entirely. This is the process of evolution.

Considered as a life-form, the Kivu Ebola isn’t a single organism but, rather, an immense swarm of particles that jumps from victim to victim. Each particle in the swarm possesses a biological drive to copy itself. As the particles copy themselves, they compete with all the other particles for survival. Ebola particles copy themselves every eighteen hours. This is the generation time of the virus—the time it takes for a particle of Ebola to get inside a human cell and potentially create thousands of identical copies of itself in the cell. The copies then exit the infected cell and drift into the bloodstream, infecting more cells. Early in the disease, Ebola patients tend to get sicker in downward lurches. In some patients, the lurches are spaced roughly eighteen hours apart, as each new generation of particles floods the body. An infected person’s bodily fluids are lethally infectious, because they are filled with Ebola particles. If some of those particles get into new people, the virus spreads.

By now, the Kivu Ebola swarm has been going through its eighteen-hour replication cycle in humans for more than a year. Some virologists wonder whether Kivu Ebola could start evolving, or whether it has already started to evolve, in a way that makes it more dangerous to people—perhaps by becoming more contagious, in which case it would get much harder to control. These questions introduce a new aspect to the international emergency.

During the Ebola epidemic that ravaged West Africa in 2014 and 2015, that form of Ebola showed possible signs of evolving. Virologists are still trying to determine the significance of what happened. The epidemic began in a village in Guinea, in December, 2013, when some particles of Ebola apparently went from a bat into a small boy. That strain of the virus, now referred to as Makona Ebola, killed the boy and most of his family, and then began spreading. In the end, around thirty thousand people were infected and more than eleven thousand died before Makona Ebola was finally brought under control and eliminated from the human population. (There were eleven cases in the United States.)

As the epidemic progressed, a team of researchers, led by Pardis Sabeti, a genomic scientist at Harvard and the Broad Institute, studied the genetic code of various samples of Ebola taken from the blood of people who had been infected. They found that the virus began mutating as soon as it got into people. “From the outset, I was intrigued by the large number of mutations we found,” Sabeti told me. Makona Ebola quickly developed into several basic varieties. Then, in late May, 2014, one of the lineages took off like a wildfire and spread rapidly all over Sierra Leone and Liberia. This lineage is named the A82V Makona Variant of Ebola. For simplicity, I’ll call it the Makona mutant. The majority of patients in the epidemic were infected with the Makona mutant, including all eleven individuals in the United States. Meanwhile, the other lineages of Ebola died out. It seemed that the Makona mutant had somehow beaten them in a contest for survival.