With Covid-19 no longer classed as a global emergency, even though it continues to kill, many experts are focusing their attention on the next danger: Disease X.
This is the as-yet-unidentified source of the next global pandemic, an event sometimes said to be a question of “when” rather than “if”.
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Disease X is not an actual condition, but instead represents the reality that, in the World Health Organisation’s words, “a serious international epidemic could be caused by a pathogen currently unknown to cause human disease”.
The WHO began to include Disease X on its list of priority diseases – those that pose the greatest risk and for which countermeasures are inadequate – in 2018, the year before the novel coronavirus emerged in the Chinese city of Wuhan.
The coronavirus has since infected hundreds of millions and caused about seven million deaths, according to official figures, although the true number is thought to be much higher.
“Clearly Covid has changed most people’s perspective in that it did happen and happened in a big way. I suppose people are concerned something could come along and be similarly or even more devastating,” said Dr Andrew Freedman, an infectious diseases specialist at Cardiff University in the UK.
Many types of pathogens could cause Disease X, although most experts, including Dr Freedman, put another coronavirus or an influenza virus at the top of their list of dangers.
In military parlance, Disease X is a “known unknown” in that it may come from an existing pathogen that, like SARS-CoV-2, which causes Covid-19, jumped the species barrier and began infecting humans.
Some pathogens that have already infected people cause scientists concern, among them the virus that causes Middle East Respiratory Syndrome, or Mers.
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The Mers coronavirus infects camels, and many of the people who have fallen ill have been those with close contact with the animals. Person-to-person transmission is rare, but the virus could mutate in a way that would make this easier.
“If there’s human-to-human transmission it’s a huge issue,” said Dr Bharat Pankhania, a senior consultant in communicable disease control and senior clinical lecturer at the University of Exeter in the UK.
Ebola and Marburg, which are both filoviruses, have very high fatality rates, but they are more easily contained because they are not respiratory viruses, said Dr Freedman.
“There have been big outbreaks, but the potential to cause a pandemic that spreads across the world is much lower,” he said.
The increase in antibiotic resistance among bacteria has been cited as a threat as big as another pandemic, with a 2019 UN report warning that by 2050 there could be 10 million deaths annually because some microorganisms cannot be tackled.
Yet this week news emerged that artificial intelligence has identified a new antibiotic, abaucin, that could be used to kill Acinetobacter baumannii, a bacterium that has been branded a superbug.
Technological breakthroughs were also much in evidence during the Covid-19 pandemic, notably with the successful emergence of mRNA vaccines, which could be designed and deployed at speed if or when another deadly pathogen – most likely viral – emerges.
“With the Covid experience, we know we can develop vaccines, so we’re in a much better place compared to 2019,” Dr Pankhania said.
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However, the risks of something “unexpected, unusual, unknown, unheard” emerging is higher than it used to be, Dr Pankhania warned, because many people live in proximity to wild animals, which are a probable source of pathogens that could spread to humans.
“There’s every possibility of something jumping from an animal into a human, then humans to humans,” Dr Pankhania said.
The continued trade in live animals in markets in China in particular remains a concern, Dr Freedman said, as do farming practices.
Compassion in World Farming, a pressure group that campaigns against intensive or factory farming, states that “the stressful, crowded conditions on factory farms help drive the emergence and spread of dangerous, infectious diseases”.
Another potential source of a pandemic is a leak from a laboratory. The Wuhan Institute of Virology, in the city where the novel coronavirus emerged, fell under the spotlight because it carried out research into coronaviruses.
There are conflicting hypotheses, so it is unlikely to ever be known whether Covid-19 was the result of a lab leak or if the coronavirus spread to people from an animal at a market in the city.
Since the Covid-19 pandemic, more countries are building biosecure laboratories to carry out work on hazardous pathogens.
A recent report by universities in the UK and US spoke of a “global boom in construction”, increasing the number of centres from which such a leak could happen.
Stringent security procedures reduce the risks, but analysts have said that a leak could be deliberate, such as if a staff member tried to launch a bioterrorism attack.
So, five years on from when the WHO first highlighted the dangers posed by Disease X, the threats remain many and varied, although the world is better equipped than ever to tackle whatever emerges.