JOHANNESBURG (AP) — The World Health Organization says the deadly Ebola virus outbreak in Congo is now an international health emergency . More than 1,600 people have died since August in the second-worst outbreak of the disease in history. Wednesday’s declaration was sparked by confirmation of a case in Goma, a Congo city of more than two million people on the border with Rwanda.

Here’s a look at Ebola and the unprecedented challenges health workers face in trying to contain what the WHO chief has called one of the world’s most dangerous diseases in one of the world’s most dangerous regions.

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WHAT IS EBOLA?

The Ebola virus can spread quickly and be fatal in up to 90% of cases. Symptoms include fever, vomiting, diarrhea, muscle pain and at times internal and external bleeding. The virus is most often spread by close contact with bodily fluids of people exhibiting symptoms and with contaminated objects such as sheets. Health care workers are often at risk.

There is no licensed Ebola treatment, but early care such as rehydration helps to improve the chances of survival. Some patients in this outbreak have received experimental treatments but their effect has not been fully studied.

An experimental Ebola vaccine has been effective in its first widespread use, and more than 163,000 people have been vaccinated. The vaccine’s testing was sped up during the West African Ebola outbreak in 2014-16 that killed more than 11,300 people.

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WHY IS THIS OUTBREAK UNIQUE?

Health workers call this the first Ebola outbreak to occur in what is essentially a war zone. Dozens of rebel groups are active in Congo’s northeast, killing hundreds of people in recent years. Attacks have led to a traumatized population that can be wary of outsiders and authorities.

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Some residents question why so much attention and money is being spent on Ebola, a disease not seen in this part of Congo until now, instead of other deadly diseases such as malaria.

Amid misunderstandings, emergency workers have struggled to explain the importance of preventative measures. An epidemiologist with WHO was shot dead earlier this year and other health workers have been attacked. The attacks have led to spikes in cases and hurt the painstaking work of tracing the thousands of people who have come into contact with those infected.

“The inability to build community trust has proven a major barrier to stopping the spread of this disease,” the International Rescue Committee’s vice president for emergencies, Bob Kitchen, said after Wednesday’s declaration. “Local communities are perplexed and frustrated by the continued increase in the number of people dying juxtaposed with a massive influx of international organizations into the region.”

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WHAT’S THE SIGNIFICANCE OF DECLARING A GLOBAL EMERGENCY?

Declaring a global health emergency often brings an increase in international attention and aid. While WHO has said that tens of millions of dollars are needed to help contain this outbreak, authorities in Congo lobbied against a declaration amid concerns that it could hurt the economy and lead other nations to close their borders.

This was the fourth time that the WHO expert committee has met on this outbreak, which some experts said met the criteria for being a global emergency months ago. For such a declaration, an outbreak must constitute a risk to other countries and require a coordinated response.

The WHO expert committee met last month after the outbreak spread into nearby Uganda . But for months, health experts have feared a spread into Goma, a major regional hub. “From here you can fly to everywhere in the world,” Dr. Harouna Djingarey, the infectious disease manager for WHO’s office in eastern Congo, said this week.

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