It's a contagion so deadly and mysterious, we know nothing about it – except that it could be the next global epidemic, according to experts at the World Health Organisation (WHO).

Code-named 'Disease X', this mystery pathogen hasn't even been discovered yet, but the looming threat of its almost certain inevitability has secured it a place on the WHO's 'most dangerous' list: a catalogue of potential future epidemics for which countermeasures are insufficient – or don't exist at all.

But how can a disease that hasn't even been identified be considered such a serious threat to public health?

The best way of thinking about it is that 'Disease X' is a placeholder for a contagious hazard we haven't encountered yet, but which is virtually certain. It's a so-called 'known unknown' that the WHO says we need to be prepared for, which is why the mystery malady is now on the agency's R&D Blueprint of priority diseases.

"Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease," the WHO explained in a recent statement.

"The R&D Blueprint explicitly seeks to enable cross-cutting R&D preparedness that is also relevant for an unknown 'Disease X' as far as possible."

The R&D Blueprint was initially released in 2015, and is reviewed on an annual basis, with the WHO explaining it exists to prioritise the top emerging pathogens likely to cause severe outbreaks in the near future, for which few or no medical countermeasures exist.

The most recent review took place in February, with experts agreeing the following diseases are the ones that most urgently require researchers' attention: Crimean-Congo haemorrhagic fever (CCHF); Ebola virus disease and Marburg virus disease; Lassa fever; Middle East respiratory syndrome coronavirus (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS); Nipah and henipaviral diseases; Rift Valley fever (RVF); and Zika virus.

This year, for the first time, the WHO added Disease X to the list, in an acknowledgment of the fact it's highly probable another pathogen will soon be added to this record – and by increasing awareness of that probability, it may actually boost research efforts to combat the imminent, unknown, threat.

"History tells us that it is likely the next big outbreak will be something we have not seen before", chief executive of the Research Council of Norway and WHO adviser John-Arne Rottingen told The Telegraph.

"It may seem strange to be adding an 'X' but the point is to make sure we prepare and plan flexibly in terms of vaccines and diagnostic tests. We want to see 'plug and play' platforms developed which will work for any, or a wide number of diseases; systems that will allow us to create countermeasures at speed."

As for where Disease X could come from, nobody knows for sure, but there a multitude of possible sources, including existing viruses that demonstrate new virulence and symptoms (such as Zika virus), engineered viruses escaped from laboratories or used as bioweapons, and zoonotic infections transferred from animals to humans.

"As the ecosystem and human habitats change there is always the risk of disease jumping from animals to humans," Rottingen said.

"It's a natural process and it is vital that we are aware and prepare. It is probably the greatest risk."

While we can be thankful Disease X probably doesn't exist yet, the likelihood of it appearing in the future is definitely something we should be aware of, in the hopes we can stay one step ahead of any future threats – especially as we continue to encounter and take over what remains of the natural environment.

"The intensity of animal and human contact is becoming much greater as the world develops," WHO scientific adviser Marion Koopmans explained to The Telegraph.

"This makes it more likely new diseases will emerge but also modern travel and trade make it much more likely they will spread."