To be around the centre of an Ebola outbreak is to become used to the smell of chlorine. At hospitals and government buildings, surfaces are sprayed with it and hands washed in a 0.05% solution that can kill the virus in 60 seconds.

Infrared handheld thermometers take temperatures at airports and border crossings. Any indication of a fever prevents passage. Contact-tracing teams crisscross the countryside.

From 2018 to 2020, Butembo, in the Democratic Republic of the Congo’s northern Kivu province, was the setting for the largest Ebola outbreak the country had seen. The complexities of the crisis were not confined to the ravages of the virus itself – they were intensified by social, political and economic pressures of an area in the midst of a conflict.

As global health officials wrestle with a serious new Ebola outbreak in the DRC, which has shocked the World Health Organization with its speed and scale, the question is what lessons have been learned from previous outbreaks?

Ebola, unlike Covid, is not a particularly efficient virus. As it is not airborne it requires physical contact with bodily fluids, including blood and vomit, to spread. That makes it particularly risky for healthcare workers, who need full-body personal protective equipment (PPE) and stringent disinfection processes.

Social practices including physical contact with the dead and dying in poor rural communities accelerated the spread in eastern Kivu and Ituri province.

A second critical factor that hampered the response six years ago was the history of political tension between the country’s government in Kinshasa and the Nande ethnic group in eastern Kivu amid an insurgency. The outbreak was exploited by cynical actors during elections, who either suggested Ebola did not exist or had been brought in by outsiders.

That, in turn, led to armed attacks, some lethal, on health workers and Ebola clinics, including one in Butembo while the Guardian was visiting.

While a new vaccination programme was available during that outbreak, there is no vaccine for the current strain of the Ituri outbreak, which is caused by the Bundibugyo variant of Ebola. It is the least well known of the three forms of the disease and has caused only two outbreaks before – in 2007 and 2012 – which killed about 30% of those infected.

Another reason for concern in the current outbreak is the suggestion that the cases may have been missed early on, potentially enabling unrecognised transmission.

One key difference from previous major outbreaks in west and central Africa is the speed with which this time the WHO has declared it a public health emergency of international concern (PHEIC).

In 2018, the WHO was roundly criticised for delaying for four months before declaring a PHEIC, defined as “an extraordinary event that may constitute a public health risk to other countries through international spread of disease and may require an international coordinated response”.

In the current outbreak, a PHEIC was declared within 48 hours, and the WHO’s head, Tedros Adhanom Ghebreyesus, said his concern was so great he had decided to act without an emergency committee meeting.

Despite that, Daniela Manno, a clinical epidemiologist at the London School of Hygiene and Tropical Medicine, has warned the current Ituri outbreak shares some of the complicating elements of the 2018 to 2020 outbreak.

“First, the number of suspected cases reported before confirmation suggests transmission may have been ongoing for several weeks before the outbreak was formally recognised,” she said.

“Second, the outbreak is occurring in a region affected by insecurity, population displacement and high population mobility, all of which can complicate surveillance, contact tracing and delivery of healthcare.

“A previous Ebola outbreak affecting North Kivu and Ituri provinces between 2018 and 2020 lasted for nearly two years, with insecurity and community mistrust repeatedly disrupting contact tracing, vaccination and response activities.

“In addition, the outbreak is now thought to be caused by Bundibugyo virus, a rare Ebola-causing virus for which there are currently no licensed vaccines or therapeutics specifically approved. There are also no vaccines in late-stage clinical development that could be readily deployed during the outbreak.

“However, it is important to emphasise that the DRC has extensive experience responding to Ebola outbreaks, and outbreak response capacity is significantly stronger today than it was a decade ago.”

Anne Cori, an associate professor in infectious disease modelling at Imperial College London, said the spread of the disease across an international border had probably influenced the quick declaration of an international public health emergency.

“A PHEIC is an official declaration made by the WHO under the international health regulations, recognising the international nature of a public health threat. It aims to help mobilise attention and resources, and coordinate response efforts at international level.

“The last PHEIC for an Ebola outbreak was declared in July 2019 during the 2018 to 2020 Ebola epidemic in the North Kivu province of the DRC. At the time, the PHEIC was declared a year into the outbreak after it reached the urban area of Goma, threatening to spread internationally to nearby Rwanda.

“The current epidemic already comprises confirmed cases across both the DRC and Uganda, which likely influenced the declaration of a PHEIC as its focus is really the international nature of the threat.”

Peter Beaumont reported from Butembo for the Guardian in 2019, visiting Ebola treatment centres and vaccination efforts.