Healthcare workers from Medecins Sans Frontieres (Doctors Without Borders) get ready to prepare isolation and treatment areas for their Ebola, hemorrhagic fever operations, in Gueckedou, Guinea in this photo taken March 28, 2014.

Given its gruesome symptoms and high fatality rate, an Ebola outbreak like the one underway now in Guinea and three neighboring West African nations inevitably raises the question: Why isn’t there a vaccine?

Health authorities have reported more than 150 cases and over 100 deaths since the outbreak was confirmed on March 21. It is centered in the remote forest region of southeast Guinea, but the disease has been found in the country’s capital, Conakry, with a population of 2 million people, and cases are suspected in Mali and confirmed in Liberia. The aid group Medecins Sans Frontieres (Doctors Without Borders), which has been on the front lines of most outbreaks since the first in 1976, is calling the current outbreak “unprecedented” because of its geographic spread.

The outbreak involves the Zaire strain of the virus, which is the most aggressive and deadly, killing 9 out of 10 people infected, according to the World Health Organization. Infection can cause sudden and severe hemorrhagic fever, with blood leaking from veins, vessels and capillaries into the intestines, bowels and respiratory system and, within days, out of every orifice in the body. Progression is swift, painful—and terrifying to both victims and witnesses.

The lethality of the virus, the lack of effective treatment and the widespread fear caused by outbreaks make compelling arguments for a vaccine, said Dr. Zoe Moodie, a senior staff scientist in the Fred Hutchinson Cancer Research Center’s Vaccine and Infectious Disease Division. These characteristics have also put Ebola on the U.S. government’s list of potential bioterrorism risks. Some of the biggest interest in developing an Ebola vaccine has come from the U.S. Department of Defense.

Other public health arguments for a vaccine include the fact that controlling the wild animal populations that serve as reservoirs of the virus or the fruit bats suspected of being its natural host is difficult, if not impossible, Moodie said. In addition, outbreaks typically strike remote areas with little healthcare infrastructure and occurs sporadically and unpredictably.

But vaccine development in general is a time-consuming, costly and frequently frustrating endeavor. When Moodie began working for Fred Hutch in 2001, she provided statistical support for the National Institutes of Health’s Vaccine Research Center, which conducted early Ebola vaccine trials. Among the challenges specific to Ebola, she noted, are the lack of an existing model for natural immunity and the disease’s rapid progression, which leaves little chance for the host to develop natural immunity. In addition, a vaccine must contend with a high rate of viral replication.

Because outbreaks occur in remote areas, obtaining samples can be challenging. And working with the virus requires the highest level of biohazard containment precautions, including astronaut-like protective suits and helmets that have contributed to the widespread fear among villages in Guinea.

Another complication, said Moodie, is that a vaccine would have to protect against multiple Ebola viral species, a challenge similar to that posed by HIV, which has even greater viral diversity.

HIV, of course, infects far more people, and finding a safe and effective vaccine is a top priority for global health researchers, including Fred Hutch. According to WHO, 75 million people have been infected with the HIV virus since the epidemic began, and about 36 million people have died. The numbers for the much more rapidly lethal Ebola virus are much lower—several thousand since the first outbreak in 1976—in part precisely because the virus is so deadly. Lethality is not the best strategy when it comes to transmission.

“The thing with Ebola outbreaks, they’re terrifying when they occur because of the high fatality rate,” Moodie said. “But because of the high fatality, it burns through an area, and then it’s over.”

The virus is spread through infected body fluids, usually to family members, healthcare workers and those who prepare bodies for burial. Beginning with the original outbreak in Zaire, now the Democratic Republic of Congo, infections typically have arisen in remote areas, probably after a hunter or a villager came in contact with or ate a monkey or bat harboring the virus. The remote locations made it hard to reach and treat victims, but also helped keep the virus locally contained.

Which explains why finding the disease in a crowded city and in multiple countries is causing concern.

For now, WHO has found that rapid response at the source of the outbreak is the best way to contain the disease. It recently delivered 3.5 tons of protection material to the country, including personal protection equipment, disinfectants and secure burial materials.