The world has shifted to the post-pandemic era since the World Health Organization (WHO) announced the end of COVID-19 as a public health emergency earlier this month. However, experts worry that a more disturbing outbreak could happen next.

Scientists have recently expressed concern over a phenomenon called "Disease X." The term is said to be a placeholder the WHO used to describe a disease that's unknown to the medical community but could cause human infections, the New York Post reported.

After Ebola, SARS and COVID-19, experts anticipate that Disease X could lead to the worst pandemic ever. Considering that it will be a new illness, there is no sure way to anticipate its causative agent. Hence, it could be a virus, a bacterium, a fungus, or other particles. What's worse is there won't be treatments available by the time it arrives.

"This isn't the stuff of science fiction. This is a scenario we have to prepare for. This is Disease X," the Coalition for Epidemic Preparedness Innovations chief executive Dr. Richard Hatchett told the Telegraph last week.

"Epidemic disease is a global threat that requires collective action. We cannot prevent new pathogens from emerging but together we can prevent the devastation they cause. Because if we know one thing, it is that Disease X, when it strikes, will not respect borders," he added.

Meanwhile, Dr. Pranab Chatterjee, a researcher at the Department of International Health at Johns Hopkins Bloomberg School of Public Health in Baltimore, said Disease X could arrive sooner than expected.

"Disease X event just around the corner. The recent spate of H5N1 bird flu cases in Cambodia is just a case in point," Chatterjee told the National Post.

Like COVID-19 and other previous outbreaks, Disease X, which was coined by the WHO in 2018, could come from animals. Public health experts believe a spillover could cause a zoonotic outbreak, which could lead to another pandemic.

If not zoonotic, bioterrorism could be the starting point of the next pandemic. An engineered pathogen is possible amid conflicts. A 2021 report published in the journal Infection Control & Hospital Epidemiology mentioned how this could happen.

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"While emerging zoonotic pathogens are a threat that needs to be monitored, the possibility of an engineered pandemic pathogen also cannot be ignored. The release of such pathogens, either through laboratory accidents or as an act of bioterrorism, might lead to a disastrous Disease X as well and has been remarked as a global catastrophic risk," the report said.

In response to this threat, medical experts worldwide have been asking for an increase in funds to support their global efforts in monitoring potential pandemic agents.