At least 10 health-care workers have been infected with the deadly Ebola virus as they battle an outbreak in an eastern province of Congo, officials said over the weekend, as concerns mount that the number of cases is growing faster than public health officials can respond.

The Congolese Health Ministry said Friday that at least 90 people had been infected in several regions of North Kivu province and neighboring Ituri province. Forty-nine people have died, including one of the 10 health-care workers who had been infected.

ADVERTISEMENT The Health Ministry and the World Health Organization (WHO) expect more cases to emerge in the coming weeks. The Ebola virus disease carries an incubation period of up to 21 days, meaning it can take as long as three weeks for an infected person to show symptoms.

Tarik Jasarevic, a spokesman for the WHO in Geneva, told The Hill that health officials had identified more than 1,500 people who had come into contact with a possible or confirmed Ebola patient.

Health officials will monitor those contacts for three weeks to make sure they are quickly cared for — or quarantined — if they show symptoms.

But, Jasarevic said, the full number of potential contacts are not yet known.

"We're at a critical moment where we are not yet sure we have all the chains of transmission identified," he said.

Those tasked with contact tracing, including employees of the Ministry of Health, the WHO and other nongovernmental organizations like Doctors Without Borders, face a challenge in the North Kivu province unlike any they have faced during previous Ebola outbreaks. The province is home to about 8 million people, including as many as a million internally displaced people who live in refugee camps, after fleeing their homes in the face of clashes between rival ethnic groups.

North Kivu province sits on the border with Uganda, and the region's largest city, Beni, has been hit by bombings carried out by Islamic militants from across the border.

Jasarevic acknowledged the uncertain security situation has meant that contact tracers cannot reach every corner of the province. Those "blind spots," he warned, could give the virus a chance to spread.

WHO has sent virus-hunters to Beni and Mangina. Doctors Without Borders last week opened an Ebola treatment center in Mangina. On Thursday , the group said it was treating 37 patients at the center — including some of those health-care workers.

"Among our patients, we have several colleagues from the Congolese health system," said Gwenola Seroux, the Doctors Without Borders official coordinating the response to the outbreak. "They were the first to respond, and some were exposed to the virus."

Anne Rimoin, an epidemiologist at UCLA, said the fact that health-care workers had been infected is a significant and troubling development. In Guinea, Liberia and Sierra Leone, where an outbreak that began in 2014 killed at least 11,300 people, already vulnerable public health systems were decimated as doctors, nurses, midwives and other health professionals came down with the virus.

"Health-care workers are at the front line and extremely vulnerable to infection. They work in poor conditions, often without personal protective equipment, and thus [are] often exposed before an outbreak is detected," Rimoin said. "The reason this is so important is because health-care workers can easily propagate disease given that they have contact with many sick people and their own families."

The Ministry of Health and other responders have begun administering a vaccine meant to protect against the Ebola virus, and health-care workers are the first to receive those vaccines. None of the more than 3,000 people who received the vaccine in another outbreak earlier this year, in Congo's Equateur province, came down with the virus, a promising sign.

A team of researchers, based in Kinshasa and overseen by Rimoin, is in North Kivu province keeping tabs on the vaccine's efficacy.

The outbreak in North Kivu is already significantly worse than the one that hit Equateur earlier this year. By the time the Ministry of Health declared that outbreak contained in July , 53 people had contracted the Ebola virus, and 29 had died.

Only days after the end of the outbreak in Equateur, Congolese health officials alerted the WHO that at least 20 people had already died from the new outbreak in North Kivu. The virus likely started to spread after the funeral of one of the first victims, a woman in Mangina. Several relatives who attended the funeral, and who probably washed and dressed the body for the afterlife, began showing symptoms just days later.

This is the 10th outbreak of the Ebola virus disease in Congo since 1976, when the virus was first identified in Yambuku, a remote village that sits on the Ebola River.