The past few weeks have brought with them unsettling news from both East Africa and Central Africa, as Equatorial Guinea and Tanzania have independently announced outbreaks of deadly Marburg virus. The first outbreak, which began in mid-February in Equatorial Guinea, has thus far consisted of at least 29 confirmed and probable cases, 27 of which have proved fatal. Tanzania’s Marburg outbreak was announced on March 21—so far, eight people have been symptomatic for the disease, and five have died. And the Centers for Disease Control and Prevention is asking American doctors to be on the lookout for cases stateside, though the risk of the virus in the U.S. is low.

Marburg virus may not be as terrifying as it was portrayed in 1994’s The Hot Zone, but any outbreak is concerning to infectious disease experts. First described in 1967 in Marburg, Germany, it is a deadly hemorrhagic fever in the same family as Ebola. The virus infects Egyptian fruit bats and can spill over to humans who come into close contact with the animals or their excretions. A person can also contract the virus from someone who’s infected if they come into contact with body fluids, contaminated clothes or bedding, or someone who has died from the virus. A Marburg virus infection is fatal in between 20 and 90 percent of all cases.

Based on these characteristics, you might think that humans would have figured out a vaccine for Marburg yesterday. You’d be partially right: Although a handful of research organizations are working independently on vaccine candidates, no vaccine has been approved to prevent viral infection, and no antiviral has been approved to treat it. Two of these vaccine candidates—made by Janssen Pharmaceuticals and the Sabin Vaccine Institute, respectively—have been tested in humans, but neither has been put through the typical wringer of safety and efficacy trials. And, as both outbreaks come under control, researchers may have missed their chance to deploy and test promising vaccine candidates.

There are several reasons why we haven’t yet come up with an approved vaccine for Marburg, Robert Cross, a virologist at Galveston National Laboratory, told The Daily Beast in an email.

One of the most salient is that “there really have not been that many outbreaks,” said Cross. “However, as we all know, when these outbreaks occur, they come with extremely dire outcomes, often with many dead.” It’s a blessing and a curse for public health researchers that Marburg outbreaks have historically been few and far in between, as a vaccine can’t be tested if people are never infected.Few cases means there are financial incentives for drug companies to want to make a vaccine.