US, May 18 -- The World Health Organization has declared the current Ebola outbreak in the Democratic Republic of Congo and Uganda a global public health emergency after the virus crossed borders into Kampala and cases surged in eastern Congo. Officials say the outbreak involves the rare Bundibugyo strain of Ebola - one of the least understood and most difficult versions of the disease to fight.

What Makes This Ebola Outbreak Different? Unlike the better-known Zaire strain responsible for the massive West African epidemic of 2014-2016, the current outbreak is caused by Bundibugyo ebolavirus, a rare subtype first identified in Uganda in 2007.

Health officials are especially alarmed because:

There are no fully approved vaccines specific...