John-Arne Rottingen, chief executive of the Research Council of Norway and adviser to the World Health Organization, told the Telegraph: 'History tells us that it is likely the next big outbreak will be something we have not seen before'.
Mr Rottingen explained Disease X could spark an epidemic if it was passed from animals to humans like HIV, which jumped from chimpanzees to humans claiming the lives of 35million people since the 1980s.
It added Disease X to the list after an annual review carried out from February 6-7, warning that there is an "urgent need" for accelerated research and development for the maladies on the list. In fact, it's not actually "real" in the strict sense of the word - Disease X is a hypothetical virus.
The organization also considered adding a number of other diseases to its list of global threats, including arenaviral hemorrhagic fevers, Chikungunya, a viral disease transmitted to humans by infected mosquitoes, and highly pathogenic coronaviral diseases.
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. Ebola is also a zoonosis.
"It's a natural process and it is vital that we are aware and prepare".
This week's brazen use of nerve gas in Salisbury, UK, underlines just how far the 100-year taboo on the use of chemical and biological weapons has broken down.
"It is probably the greatest risk".
Given the rapid development of gene-editing technologies, Disease X could also spring up from human error or malevolence - in which case, having a flexible, widely-applicable plan of action is of paramount importance.
Whatever the case may be, the WHO hopes its list will spur governments across the globe to invest more into strengthening local health systems.
The illness - now given the name Disease X - has the potential to spread quickly, and any cures or countermeasures are now severely lacking.
World Health Organization said existing drugs and vaccines need further improvement for several of the diseases considered but not included in the priority list. These classes will be considered for inclusion on next year's list.