The illness - now given the name Disease X - has the potential to spread quickly, and any cures or countermeasures are now severely lacking. "They should be watched carefully and considered again at the next annual review".
For the purposes of the R&D Blueprint, the WHO developed a special tool for determining which diseases and pathogens to prioritize for research and development in public health emergency contexts.
It was the first time that Disease X made the list.
This tool seeks to identify those diseases that pose a public health risk due to their epidemic potential and for which there are no, or insufficient, countermeasures.
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.
John-Arne Rottingen of the Research Council of Norway tells the Telegraph that "history tells us that it is likely the next big outbreak will be something we have not seen before". According to WHO, "Disease X" will be caused by a pathogen which is not yet identified to cause human diseases.
The mysterious nature of the disease is meant to ensure flexible planning of diagnostics tests and vaccine strategies, so that that they may be applied to a wide range of possible scenarios.
"It's a natural process and it is vital that we are aware and prepare. It is probably the greatest risk", Mr. Rotingen adds.
The WHO hopes to improve disease surveillance and the capacity of local health systems, which would help to detect an outbreak quickly.
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.
World Health Organization said existing drugs and vaccines need further improvement for several of the diseases considered but not included in the priority list.