The Ebola outbreak in the Democratic Republic of the Congo and Uganda is of particular concern to global health officials in part because the virus detected is a less common strain. 

As of Sunday, when the World Health Organization declared a public health emergency, there were nearly 250 suspected cases and 80 suspected deaths. These numbers are expected to rise, as there are "significant uncertainties to the true number of infected persons and geographic spread," the WHO said.

The virus behind this outbreak is the Bundibugyo virus, health officials have confirmed. This is only the third known outbreak of this virus, and there are no vaccines or treatments.

Ebola vaccine targets Zaire strain

Ebola disease is caused by orthoebolaviruses, of which there are multiple species. 

Three of the viruses have been known to cause large Ebola disease outbreaks, according to the WHO. They are Ebola (or Zaire) virus, which has been the most common; Sudan virus; and Bundibugyo virus, the one identified in this outbreak.

The only approved vaccine and treatments are for the Zaire strain. Because each virus species has different genetic material, they need different vaccines. 

There are some other vaccines in development, but nothing that targets Bundibugyo virus is close to being ready for use, Dr. Céline Gounder, a CBS News medical correspondent, said. 

2 previous Bundibugyo outbreaks

Before this outbreak, there were two known outbreaks caused by the Bundibugyo virus, and both were smaller than the current one. 

The first discovery of the Bundibugyo strain occurred in the Bundibugyo District in Uganda in 2007. There were 149 cases and 37 deaths, The Associated Press reported.

The second known outbreak was in Congo in 2012, with 57 cases and 29 deaths reported, according to the AP.

Because there have been fewer outbreaks of this virus, there is far less data about how it behaves than for the Zaire virus, which has been around since 1976 and caused dozens of outbreaks, Gounder said. 

Symptoms of Ebola disease

Bundibugyo virus disease, the type of Ebola disease caused by the Bundibugyo virus, is severe and often fatal. The virus spreads from person to person through direct contact with bodily fluids of someone who is sick or has died from the disease.

The early symptoms can often be mistaken for other illnesses, the WHO says. They include:

• fever
• fatigue
• muscle pain
• headache
• sore throat

Those can progress to other symptoms, including:

• vomiting
• diarrhea
• abdominal pain
• rash
• organ dysfunction
• internal or external bleeding (less frequent)

The fatality rate of Bundibugyo virus disease, based on the past two outbreaks, is about 30 to 50%, the WHO says. While still highly deadly, this is lower than the fatality rate of the Zaire strain, which can be up to 90%.

"Early intensive supportive care including rehydration and treatment of specific symptoms, can improve survival," the WHO says. "Seeking early care can be lifesaving."