An Ebola outbreak in the Democratic Republic of Congo has been declared a public health emergency of international concern, by the World Health Organization (WHO).

The latest outbreak is challenging because it involves a rare strain of Ebola for which there is no vaccine, and the epicentre is in an area affected by conflict.

The WHO has told the BBC that the outbreak may be spreading faster than originally thought.

Ebola is a rare but deadly disease caused by a virus.

Ebola viruses normally infect animals, typically fruit bats, but outbreaks among humans can sometimes start when people eat or handle infected animals.

It takes two to 21 days for symptoms to appear. They come on suddenly and start like the flu or malaria, with fever, headache and tiredness.

As the disease progresses, vomiting and diarrhoea develop and it can lead to organ failure. Some, but not all, patients develop internal and external bleeding.

The virus spreads from one person to another by contact with infected bodily fluids such as blood or vomit.

Ebola outbreaks used to be small and contained to remote rural areas. However, urbanisation is pushing larger populations closer to these natural reservoirs of Ebola and increasing the risk of transmission.

This outbreak is caused by the rare Bundibugyo species of Ebola, which had not been seen for over a decade.

Named after a district in Uganda where it was first detected, Bundibugyo has only caused two previous outbreaks – in 2007 and 2012.

One study showed that it killed about a third of those infected, far less than the more common Zaire (66.6%) and Sudan (48.5%) strains.

Initial blood tests for Ebola in the affected areas were negative as they were designed to identify the more common species of the disease.

There is no approved vaccine for Bundibugyo, but experimental ones are in development. It is possible that a vaccine for the Zaire species may offer some protection.

There are also no drugs that target Bundibugyo, making it harder to treat.

A further complication is that the outbreak is taking place in a conflict zone, with a quarter of million people displaced from their homes and people moving across porous borders into neighbouring countries.

Trish Newport, from medical charity Doctors Without Borders, who is heavily involved in efforts to tackle the outbreak, told the BBC World Service that territory constantly changed hands between different armed groups, making it difficult for emergency response teams to simply drive to Ebola hot-spots.

She pointed out that a further problem was bad roads, with a 90km (56-mile) journey from Bunia city to Mongwalu, one of two gold-mining towns where the majority of cases have been reported, taking more than three hours.

Newport added that for Mongwalu's residents, the Ebola outbreak was a "very scary moment".

In some families, 15 people had died and some people had fled the town, she said.

However, the WHO's declaration of a public health emergency of international concern does not mean we are in the early stages of a Covid-style pandemic. The risk Ebola poses outside Central and East Africa is minimal.

The first known case was a nurse who developed symptoms on 24 April, which means the virus had been spreading undetected for weeks.

The nurse died in Bunia, the capital of eastern DR Congo's Ituri province, according to Congolese Health Minister Samuel Roger Kamba.

The victim's body was repatriated to Mongwalu.

Kamba said one of the reasons the virus spread so quickly was the number of people exposed to the body during the funeral ceremony.

The Africa Centres for Disease Control and Prevention told the BBC World Service that funerals were a particular concern, as they also helped spread the disease during previous outbreaks.

Director Dr Jean Kaseya said public health information campaigns were "providing information on how to handle funerals" and the importance of basic hygiene and sanitation, as well as providing protection measures for health workers.

Kamba said there have been delays in reporting Ebola cases because infected communities believe the disease to be "witchcraft" or a "mystical illness", resulting in people seeking treatment from prayer centres and witchdoctors rather than hospitals.

WHO chief Tedros Adhanom Ghebreyesus said he was "deeply concerned about the scale and speed of the epidemic".

As of 20 May, officials said 600 cases were suspected in DR Congo, where at least 139 people are thought to have died from the virus. Another person died in Kampala, the capital of neighbouring Uganda after travelling from DR Congo.

But modelling by the London-based MRC Centre for Global Infectious Disease Analysis released on 18 May suggested there had been "substantial" under-detection, and there could be more than 1,000 active cases.

The 51 cases confirmed in DR Congo are in its eastern Ituri province – the epicentre of the outbreak – and the North Kivu province.

American doctor Peter Stafford tested positive after treating patients at Nyankunde Hospital in Bunia, where he has worked since 2023.

The US Centers for Disease Control and Prevention (CDC) said he had been evacuated to Germany for treatment, and that it was working to evacuate at least six other Americans who had also been exposed to the disease.

A case has also been confirmed in eastern DR Congo's biggest city, Goma. It has a population of around 850,000 and is controlled by rebels from the AFC-M23 group.

The patient travelled to the city after her husband died of Ebola in Bunia, Jean-Jacques Muyembe, the director of the Congolese Institut National de Recherche Biomédicale (INRB), told AFP news agency.

Another person is still being treated in Uganda. The two cases identified there are both Congolese nationals who had recently travelled to the country.

The Congolese government has sent health teams to Bunia with protective equipment.

The WHO and Doctors Without Borders are setting up treatment centres and working on a response plan.

The WHO has dedicated $3.9m (£2.9m) to tackling the outbreak.

A toll-free number, 151, has been provided for reporting symptoms.

Residents have been urged to take measures such as:

• calling immediately when symptoms appear
• avoiding contact with bodies of people who died with symptoms, or with dead animals
• not eating raw meat, as undercooked food may transmit the virus
• practising social distancing

The AFC-M23 group says it is creating an Ebola response team to prevent the spread of the disease in the areas it controls.

On 17 May, spokesman Lawrence Kanyuka said the group had "immediately activated" response mechanisms in conjunction with health services and local medical facilities.

Neither the government nor the rebels have explicitly said whether they are prepared to work together to tackle the outbreak.

However, the case in Goma was confirmed by a state-run body, the INRB.

Caitlin Brady, the country director for the Danish Refugee Council, is currently in Goma to prepare her organisation's response. She says she has been informed by the rebels that they are using contact-tracing and all appropriate measures to contain the virus.

She told the BBC World Service's Newsday programme that "a lot of the health officials and healthcare workers stayed and continued working" after rebels seized the city, meaning "the capacity to respond has remained".

Africa CDC has warned of the high risk to countries that border DR Congo, specifically Uganda, Rwanda and South Sudan.

Several African countries are tightening border screenings and bolstering health facilities.

Rwanda has closed its borders with DR Congo.

Uganda has told people to avoid hugging and shaking hands. President Yoweri Museveni also postponed the Martyrs' Day pilgrimage, a Christian holiday held on 3 June each year, which usually draws thousands of Congolese nationals to join festivities.