The World Health Organization (WHO) says there have now been 600 suspected cases of Ebola and 139 suspected deaths, with numbers expected to rise further given the time taken to detect the virus.
Fifty-one cases have now been confirmed in the Democratic Republic of Congo – where the first case was reported – and two in neighbouring Uganda, WHO chief Dr Tedros Adhanom Ghebreyesus said on Wednesday.
Speaking to journalists in Geneva, he said the outbreak of the Bundibugyo species of Ebola was likely to have started "a couple of months ago".
On Sunday, the WHO declared a public health emergency of international concern, but said it was not at pandemic level.
Ghebreyesus said that after meeting on Tuesday, the health organisation's emergency committee agreed the situation was "not a pandemic emergency".
"WHO assesses the risk of the epidemic as high at the national and regional levels and low at the global level," he explained.
The 51 cases confirmed in DR Congo are in its eastern Ituri province – the epicentre of the outbreak – and the North Kivu province. Of the two confirmed in Uganda's capital, Kampala, both had travelled from DR Congo, one of whom has died.
"We know the scale of the epidemic in DRC is much larger," the WHO chief said, adding that healthcare workers were among those who had died, which was a particular concern.
Local health workers say some facilities are being overwhelmed. Although personal protective equipment has started to arrive, they say they are still working without adequate protection.
A WHO official said investigations were under way to find out how long the virus had been spreading for, but that their priority was to curb transmission.
The first known case was a nurse who developed symptoms and died on 24 April, in Ituri's provincial capital Bunia.
The body was repatriated to Mongwalu, one of two gold-mining towns where the majority of cases have been reported.
Four areas in Ituri have been affected by the outbreak: Mongwalu, Bunia, Rwampara and Nyakunde.
In North Kivu, rebel-controlled Goma, eastern DR Congo's biggest city and Butembo have reported cases.
Araali Bagamba, a lecturer who lives in Bunia, said people understand how dangerous the situation is.
"For the last three days I haven't shaken anyone's hand and I observe that within the general population," she told the BBC World Service Newsday programme. "It's our habit to shake hands all the time… [but] the habit has changed."
Ebola is spread through direct contact with bodily fluids and through broken skin, causing severe bleeding and organ failure.
Bagamba said people "believe it will get worse before it gets better", because people did not initially realise it was Ebola.
Ebola was first discovered in 1976 in what is now DR Congo, and is thought to have spread from bats.
There are four species of Ebola known to cause disease in humans, including Zaire, which DR Congo has dealt with on numerous occasions and is most familiar with.
The country is facing its 17th outbreak of Ebola, but the Bundibugyo species – which has not been seen for more than a decade – brings its own difficulties.
Bundibugyo has only caused two previous outbreaks – in Uganda in 2007 and DR Congo in 2012 – when it killed about a third of those infected.
Although less deadly than other Ebola species, the rarity of Bundibugyo means there are fewer tools to stop it.
There is no approved vaccine for Bundibugyo, but experimental ones are in development. It is possible that a vaccine for Zaire may offer some protection.
There are also no drugs that target Bundibugyo, making it harder to treat.
Following criticism from the US on Tuesday that the WHO was "a little late" in identifying the outbreak, Ghebreyesus said these comments might have been caused by a lack of understanding.
"We should appreciate what was done so fast in a highly complex setting," the WHO said.
Initial symptoms of Ebola mirror illnesses such as malaria and typhoid, which are common in DR Congo.
Eastern DR Congo is also badly hit by years of conflict, bringing additional difficulties in dealing with the virus.
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