In 1969, an American missionary nurse named Laura Wine came down with a troubling fever while working in the Nigerian town of Lassa. The local doctors thought it was probably malaria, but Wine didn’t respond to the usual treatments. She eventually died. Shortly after, two more nurses contracted the same mysterious disease. One also died. The other, Lily Pinneo, was evacuated to Columbia-Presbyterian Hospital, and survived. From her blood, and those of her colleagues, scientists isolated a new virus, which they named after the town where the infections began.

Since then, scientists have learned a lot about Lassa fever, and the virus that causes it. They discovered that it resides within the multimammate mouse and jumps into people who eat food contaminated by the rodent’s waste. They’ve shown that it is common in West Africa, and causes many thousands of cases every year.

But for all that knowledge, no one knows exactly why this disease, which simmers gently in Nigeria from year to year, has recently come to a dramatic boil.

It’s hard to say exactly who has Lassa fever, because early symptoms are generic, and resemble conditions like malaria and typhoid. But according to figures from the Nigerian Center for Disease Control, laboratory tests have confirmed that in the first two months of 2018, at least 317 people have been infected, and at least 64—around 20 percent—have died. (Eight further deaths are probably linked to Lassa, but haven’t been confirmed.) By contrast, there were just 143 lab-confirmed cases in all of 2017, and just 101 in 2016.

Lassa fever has similarities to Ebola, in that severe infections are accompanied by bleeding from the mouth, nose, and gut. But Lassa is both less contagious and less deadly than its infamous counterpart. Around 80 percent of infected people don’t develop symptoms at all, and those that do experience the usual litany of fever, vomiting, fatigue, and aches. More worryingly, around a quarter of survivors become deaf, although half of those regain some degree of hearing.

The patients of 2018 aren’t any sicker than normal, says Christian Happi, who directs the infectious-diseases lab at Irrua Specialist Teaching Hospital in Nigeria. It’s just that there are more of them. And it’s not clear why.

In addition to jumping from rodents to humans, Lassa can spread from person to person through infected bodily fluids, though such transmissions are rare and usually limited to health-care workers. So far, there’s no indication that the virus has become more transmissible, or has changed in other important ways. But researchers from the Broad Institute of MIT and Harvard will know for sure once they sequence the genes of Lassa samples, which they’ve been collecting for years. “Is it a slightly different strain of the virus? Is it transmitting differently? We’ll have a lot more clarity once we see the genomics of the virus,” says Kayla Barnes, who studies how pathogens evolve at Harvard.

Another possibility is that the number of Lassa infections isn’t actually going up; it’s just that Nigeria is getting better at spotting them. “When you don’t have the ability to detect the disease, you’re not going to find it,” says Lina Moses, from Tulane University. In recent years, Nigeria has invested money in diagnostic tests that can confirm Lassa infections, and media coverage has raised the profile of the disease. “Clinicians are better at keeping Lassa at the back of their minds, and in ordering the tests,” says Moses. “It’s not at all surprising to me to see an increase in cases. It’s likely a good sign that the health system is improving.”

But Daniel Park, from the Broad Institute, says this can’t be the sole explanation, as there genuinely are more sick people to care for. “There’s a felt difference in the disease burden on the ground,” he says.

Changing climate and Nigeria’s growing population may also be a factor. As agricultural land expands, the mouse that carries Lassa virus is forced out of fields and moves into human homes. Its population also varies with the climate, booming after periods of rain. When the rains end, the rodents’ food supplies dwindle, compelling them to scurry into human homes in search of a meal. In a simulation of environmental changes, Moses and her colleagues predicted that Lassa spillovers are likely to double by 2070. But “there’s so much about the ecology of Lassa in rodents, and the rodent’s relationship with humans, that we don’t know,” she says.

The fact that Lassa’s animal host is a mouse makes it especially hard to contain. The rodents are common, inconspicuous, comfortable in human settlements, and attracted to the food we eat—all of which increase the odds of a viral spillover. Also, “people enjoy eating them,” says Happi. “They’re the major sources of protein for some communities. If you tell them not to eat these rats, what are you going to give them instead?”

Lassa fever’s mildness, and its long incubation time of one to three weeks, also means that it travels easily. “People feel bad for a few days, not horrible, so they’re more likely to get on a plane,” says Moses. For that reason, Lassa is one of the most commonly exported viral hemorrhagic fevers. It has made its way to the United States on six recorded occasions, the latest in 2015. Fortunately, unlike Ebola, Lassa doesn’t spread easily from person to person, so its ability to cause a full-blown pandemic is limited.

Still, it is not a problem to be taken lightly. Lassa can be treated with an antiviral drug called ribavirin, but that needs to be given during the first week of infection. It also comes with several side effects. “It’s a fairly harsh therapy,” says Barnes. “It’s not a super effective treatment, but there’s nothing else. There’s been a recent push in the scientific community to develop a better drug.” Researchers are also trying to develop a vaccine. In 2017, various governments and nonprofits created the Coalition for Epidemic Preparedness Innovations—a global fund to accelerate the development of vaccines against important infectious threats. They picked three initial diseases to target, and Lassa was one of them.

In Nigeria, investments from the United States and other wealthy nations have already been crucial in transforming a few hospitals into centers where Lassa can be accurately diagnosed. That allows doctors and nurses to take the right precautions, minimizing the already small risk of contracting the virus from their patients. Happi’s hospital in Irrua “used to lose at least five medical practitioners a year to Lassa,” he says. “Now, it hasn’t lost anyone to Lassa in 10 years, because we diagnose and treat them. That shows how foreign investment can change health. A small hospital in a rural community can become a center of excellence for treating and diagnosing Lassa fever, in a country of almost 200 million people.”

Still, the Irrua hospital is just one of three facilities in the country that can diagnose Lassa, and it is currently facing a shortage of personal protective equipment, like face masks and goggles. Nigeria suffers from the same cycle of panic and neglect that stops most countries from fully preparing for epidemics. “It seems to me that after each outbreak, people go to sleep,” says Happi. “And then we go back to this whole process of screaming and shouting.”