The organisation released a list of diseases it considers posing a high risk to the public due to their potential to spark an epidemic and the limited treatment available to combat them.
This came from an expert committee set up by the organisation, to identify pathogens with the ability to spread and kill many people without any available remedies.
Viruses such as Ebola, Zika, Lassa fever and Severe Acute Respiratory Syndrome, which have raged in recent years, are included as serious threats.
However, WHO has included Disease X in its priority list for the first time this year after a review by health experts in February.
Disease X is not a newly discovered threat, but an imaginary virus, which could emerge in the future and cause widespread infection across the globe.
“Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease,” WHO said in a statement.
It added that healthcare officials were planning for a currently unidentified threat now to ensure research and development preparedness.
“History tells us that it is likely the next big outbreak will be something we have not seen before”, John-Arne Rottingen, chief executive of the Research Council of Norway and a scientific adviser to the WHO committee, told The Telegraph of the UK
“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.”
Rottingen said Disease X could come from a variety of sources, although it was most likely developed through zoonotic transmission, where an infectious disease, which usually afflicted animals, jumped to humans.
Ebola and salmonella infection are both zoonosis, as well as HIV, thought to have been transferred to humans from chimpanzees and monkeys in the early 20th Century.