JOHANNESBURG (AP) — More than 1,000 deaths have now been confirmed in eastern Congo's Ebola outbreak, the second-worst in history behind the West African one in 2014-16 that killed more than 11,300 people. Attacks on health workers are badly complicating efforts to contain one of the world's most notorious diseases. At least 85 have been wounded or killed since January.

Here's a look at why the outbreak is especially challenging, even as a promising experimental vaccine is being used widely for the first time. Nervous health experts watch as the virus spreads in a dense, highly mobile population near the border with Uganda and Rwanda.

"An under-resourced operation creates a very real risk of an international spread of Ebola," Nicole Fassina with the International Federation of Red Cross and Red Crescent Societies said Saturday. A $54 million funding gap exists, the World Health Organization says.

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WHAT IS EBOLA?

Ebola is a virus that can spread quickly and can be fatal in up to 90% of cases. Symptoms include fever, vomiting, diarrhea, muscle pain and at times internal and external bleeding and can start to occur between two and 21 days from infection, according to WHO.

At least 1,529 confirmed and probable Ebola cases have been reported in this outbreak declared in August, with 1,008 deaths.

The virus is most often spread by close contact with bodily fluids of people exhibiting symptoms and with objects such as sheets that have been contaminated. Health care workers are often at risk, and burial practices that call for close contact with victims can spread the disease.

While there is no licensed treatment for Ebola, early care such as rehydration and treatment of other symptoms helps to improve survival chances. Experimental treatments have been given to some patients in this outbreak but their effect is yet to be fully studied.

An experimental Ebola vaccine has been effective, and WHO says a second one might be introduced. Congo's health ministry says more than 110,000 people have been vaccinated, but some including health workers have refused it.

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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.

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

Amid rumors and misunderstandings, health workers struggle to explain the importance of safe burials and other preventative measures. One out of four people interviewed in eastern Congo a month after the outbreak began wrongly believed Ebola wasn't real .

Some people have fled, or attacked health workers. An epidemiologist with WHO was shot dead last month and some doctors threatened to go on strike indefinitely if health workers are attacked again.

The attacks have led to spikes in cases and hurt the painstaking work of tracing contacts of infected people. More than 12,000 people are tracked per day, WHO says. And yet many new cases are unconnected with known ones and many people are dying at home.

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HOW LONG WILL THIS OUTBREAK LAST?

It's hard to say. Some health experts once predicted that the outbreak would end within six months' time. As complications keep appearing, the time frame is pushed back.

The chair of WHO's expert committee that unanimously decided last month — for a second time — that the outbreak is not yet a global emergency has said experts were "moderately optimistic" the outbreak could be contained within a "foreseeable time."

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