Over 90 people have died so far in West Africa’s ebola outbreak, which Doctors Without Borders (also known as MSF) is calling “unprecedented.” And the deadly disease, reports indicate, is continuing to spread.

Ebola’s a frightening disease: It’s one of the world’s most lethal viruses, and the CDC ranks it among anthrax and smallpox as a Category A bioterrorism agent. As some have pointed out, however, other diseases are taking a much bigger toll in the developing world. Ebola has killed 1,500 people in total since it was first documented in 1976. But the dramatic nature of Ebola aside, health workers say there are a number reasons to believe this latest outbreak is particularly concerning:

It’s widespread.

The outbreak began in Guinea two months ago, and has since crossed international borders to Sierra Leone and Liberia, where it’s so far killed seven people. Now, Mali, another neighboring country, is reporting its first suspected cases as well.

Because Ebola isn’t airborne — it can be transmitted only through direct contact with the blood or body secretions of someone who’s infected — containing it is theoretically simple: health workers need to identify all possible cases and quarantine them. The widespread distribution of the outbreak complicates this, as does the fact that it’s being found in urban centers instead of remote, rural areas.

“We are facing an epidemic of a magnitude never before seen in terms of the distribution of cases in the country,” said Mariano Lugli, the coordinator of MSF’s project in Conakry, Guinea’s capital. “MSF has intervened in almost all reported ebola outbreaks in recent years, but they were much more geographically contained and involved more remote locations. This geographical spread is worrisome because it will greatly complicate the tasks of the organizations working to control the epidemic.”

Guinea is unprepared.

“This is the first time ebola is detected in Guinea, so the population and the medical staff don’t know the disease,” Esther Sterk, a tropical medicine adviser for Doctors Without Borders, explained to NPR. At least 11 health care workers are among those infected, mostly likely because, as one expert explains, “they didn’t know what they were dealing with.”

There’s also a large amount of stigma and fear associated with ebola, Laurie Garrett, a senior fellow with the Council on Foreign Relations, explained to Bloomberg News. That attitude could cause patients to seek care in hospitals far away from their local communities, further spreading the disease. “If those hospitals are not aware of what’s coming,” Garrett added, “they will quickly become cauldrons, and spread the virus internally.” To help counteract that problem, NPR reports, anthropologists are being flown into Guinea alongside health workers to help them contain the outbreak in a “culturally sensitive and appropriate” way.

The only way to protect people is to keep them from getting it in the first place.

The containment issue is so important because there’s no vaccine to protect people from ebola; there’s also no treatment or cure. Once contracted, the disease kills about 90 percent of patients.

“The people who have seen cases of ebola are really scared,” Roland Berenger, the West Africa emergency manager for aid organization Plan International, told National Geographic. ”When you see people dying, bleeding to death, and there is nothing anyone can do, you get scared.”

Lindsay Abrams is an assistant editor at Salon, focusing on all things sustainable.