The deadly Ebola virus outbreak in eastern Africa could be the “worst ever” in history, the director-general of Africa’s Centres for Disease Control and Prevention, Jean Kaseya, has warned.

Kaseya sounded the alarm on Tuesday during a virtual meeting of African heads of state and international donors in Burundi.

At least 837 people have been infected in the epicentre of the outbreak, the Democratic Republic of Congo, while 196 people have died, authorities say. In neighbouring Uganda, 19 cases and two deaths have been reported.

“If we don’t stop the outbreak very soon, it will be worse than what we had in West Africa and eastern DRC,” Kaseya said, referring to previous outbreaks of the virus in West Africa in 2014, which resulted in the deaths of more than 11,000 people, and a later one in the DRC in 2018 that led to the deaths of more than 2,000.

Here’s what we know about why health officials fear this outbreak could be even worse:

What is Ebola?

Ebola disease is a viral haemorrhagic fever that can result in death. It is spread through bodily fluids, and victims remain highly infectious after death.

Three known forms of the virus cause large outbreaks of the disease: Zaire virus, Sudan virus and the Bundibugyo virus, which has caused the current outbreak.

The Zaire strain caused previous outbreaks in the DRC and in West Africa.

Both the Zaire and the Bundibugyo strains have a fairly high death rate of between 30 percent and 50 percent.

The West Africa outbreak was the largest on record: It spread to several countries and infected nearly 29,000 people between 2014 and 2016, including nationals of Italy and the United States. Two vaccines and experimental treatments for the Zaire strain were developed.

In the 2018-2020 DRC outbreak, the second-largest, some 3,400 people were infected. However, officials vaccinated more than 300,000 and deployed approved treatment.

How is this latest outbreak different?

Although the Bundibugyo virus has appeared before – in 2007 and 2012 in eastern DRC – it is rarer than other forms of Ebola, and there are currently no approved vaccines or treatments for it, although these are in early stages of development. Vaccines which were developed to combat the Zaire form cannot simply be used to prevent Bundibugyo without World Health Organization (WHO) testing and approval.

Analysts say this strain has so far been ignored because of its rarity.

“It’s not something that attracts the attention of research and development for pharmaceutical companies, for companies that are making vaccines,” Trish Newport, deputy manager of Ebola programmes in DRC for Doctors Without Borders, known by its French initials MSF, told Al Jazeera.

“For them, they don’t see it as a priority. This is definitely one of the reasons why it wasn’t put on the priority list, why there weren’t resources for it and why it’s only being trialled right now.”

Why could this be the ‘worst ever’ outbreak of Ebola?

First, because there’s no approved vaccine or treatment for Bundibugyo, health workers have little option other than to manage symptoms of the deadly disease as they would an outbreak of flu.

This outbreak is also taking place against the backdrop of an ongoing conflict.

Eastern DRC, where the virus is spreading, has become a battlefield on which armed groups, most notably the M23 rebel group, are battling government forces despite ongoing diplomatic efforts to end the conflict. This makes it much harder for health workers to travel in the region to trace the disease’s spread effectively or provide care to infected people who might be in rebel-held territory or crowded refugee camps.

As a result, authorities say, this virus is spreading faster than healthcare workers can keep pace with, and hundreds of people had already been infected by the time the WHO declared a public health emergency in mid-May. Officials fear there are numerous undetected cases.

On Tuesday, Keseya told Al Jazeera that tens of thousands could be infected but have not been traced at all.

“The contact tracing is a major indicator and a major issue,” he said. “We are missing more than 26,000 people, and we don’t know where they are, and we don’t know if they are contaminating other people.”

Already, the virus has spread from DRC’s commercial Ituri Province to North and South Kivu, and across the border into Uganda.

According to WHO officials, cases are being detected in new areas within the affected DRC provinces on a “near-daily” basis, reflecting the scale of local community spread.

Added to all of this is the stigma and disinformation that is circulating about Ebola in communities in the DRC. Many believe the disease is a ruse for the government to siphon funds. Some refuse to report their symptoms at all due to disbelief or shame surrounding being an Ebola patient.

Communities have also voiced anger at being unable to bury their dead according to traditional rites, with anger sometimes directed at health workers. Early in June, angry youths attempting to remove their dead relatives for burial broke into a hospital and torched treatment tents and other medical infrastructure.

International intervention is more limited this time, compared with the earlier West Africa outbreak.

In 2014, international donors responded with between $5.9bn and $8.9bn of funding and personnel help. The US military even supported Liberia with a treatment centre.

This time around, Burundi’s President Evariste Ndayishimiye, who serves as the current African Union chair, says only one-fifth of the $518m needed to respond to the outbreak has so far been raised.

The latest outbreak is occurring at a time when international cooperation on health and aid is shaky, largely due to the US’s slashing of foreign aid at the start of President Donald Trump’s term and the subsequent closure of the United States Agency for International Development (USAID), which had supported many health programmes overseas. European donors have also scaled back assistance in the past year.

The local response in the DRC has also been hindered by low numbers of isolation centres, which help break the transmission chain. There is also a lack of personal protective equipment (PPE) for healthcare workers, and at least four healthcare workers have died after contracting the disease.

Newport of MSF said while funding is needed to tackle the disease, they must also continue to address non-Ebola needs to maintain locals’ trust.

“We have to ensure that people have access to non-Ebola healthcare as well, that they have access to water,” she said. “Ebola might be the priority of the funding organisations responding, but it’s not necessarily the priority of the populations there, and we need to be listening to the population.”