Health workers started vaccinating people in Uganda against Ebola virus on Wednesday, the first time the vaccine has been given in a country before an outbreak even starts.

Officials are afraid the virus is going to spread across the border from the Democratic Republic of Congo, where an outbreak is still worsening, fueled by armed conflict and frightened residents. At least 300 suspected cases of Ebola have been reported in the DRC, with 265 confirmed. Of those, 151 people have died, the World Health Organization says.

“In vaccinating frontline health workers against Ebola virus disease even before Uganda detects a single case, health authorities are being cautious having learnt bitter lessons from previous outbreaks,” the World Health Organization said in a statement.

Among the new #confirmed cases in #Kalunguta, there is a 6-day #newborn #baby from a #confirmed case who #died on November 4, 2018 in the #Butembo #ETC. The child's mother began to show symptoms of the disease 5 days before giving #birth to her son. — Katharina Kopp (@koppk_OHPGXC) November 6, 2018

The vaccine, which is still technically considered experimental, will not be given to the general public, but to people who would likely be the first to examine people suspected to have Ebola infections in areas bordering the Democratic Republic of Congo.

“It is highly likely that Uganda may import Ebola virus disease from DRC given the closeness of the current epicenter, the high population movements due to trade, social-cultural connections and easy accessibility of health services in Uganda,” the WHO said.

In the DRC, more than 26,000 people have been vaccinated in a “ring vaccination” strategy. Under this approach, all the people who have been in contact with an Ebola patient are vaccinated, and then people who have been in contact with those contacts are also vaccinated. This approach worked to eradicate smallpox in the 1970s.

Despite vaccination efforts, Ebola is still spreading in eastern Congo. It’s a war zone, with militant groups battling the government for control of various regions. The conflict threatens health workers trying to battle the virus.

“I cannot access entire villages. Not because of the unequipped terrain and unpaved dirt roads but because these villages act as front lines between the different armed groups,” Dr. Eric Mukuma, a health zone coordinator for the aid group CARE, wrote in an editorial in the Guardian newspaper.

Some patients have also fled treatment centers, carrying the virus with them. Fighting fear and mistrust has been a big problem for health workers in various Ebola outbreaks.

Ebola has popped up multiple times in Central Africa since it was first identified in 1976. The worst outbreak was in Uganda in 2000 and 2001, when 574 people were infected and 261 died. By far the worst epidemic was in West Africa in 2014-2016, when 28,000 people were infected and 11,000 died in Sierra Leone, Liberia and Guinea.

Ebola kills between 20 percent and 70 percent of victims, depending on the strain. There are four experimental treatments being tried out in DRC: ZMapp, a cocktail of antibodies that fight the virus; a second cocktail of antibodies called REGN-EB3; a single-antibody drug called mAb114; and an antiviral drug called remdesivir.

Supportive care, including replacing fluids and electrolytes lost to diarrhea and vomiting, can also help keep patients alive.

This is the second outbreak of Ebola in Congo this year. An outbreak in the western part of the country was declared over in July, after infecting 53 people and killing 33.

Just days later, the current outbreak was discovered across the vast Central African nation, in a region that has been at the heart of multiple conflicts.