The organization further says, though no individual has been identified as affected by Disease X, man-made deceases are hard to be controlled hence the public should be prepared for worse.
The World Health Organisation (WHO) has revealed details about "Disease X" in its second annual review of priority diseases.
"Disease X" is the code name for an unknown pathogen that could be created by biological mutation, such as previous deadly epidemics such as Spanish Flu or HIV, the organization said.
By placing this mystery illness on the "List of Blueprint priority diseases", alongside conditions such as MERS and the Marburg Virus, WHO acknowledges that infectious diseases can happen anytime, and are highly unpredictable.
"Disease X represents the knowledge that a serious global epidemic could be caused by a pathogen now unknown to cause human disease", World Health Organization said on its website.
Despite not being identified as a specific disease, the pathogen has been classed as extremely unsafe.
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In previous years the list has been confined to known killers such as Lassa fever, which is now sweeping Nigeria, and Ebola, which killed more than 11,000 people in an epidemic in West Africa between 2013 and 2016.
Disease X "represents the knowledge that a serious global epidemic could be caused by a pathogen now unknown to cause human disease", the Geneva-based World Health Organization said. She said that "it may seem unusual to be adding an "X" but the point is to make sure we prepare and plan flexibly in terms of vaccines and diagnostic tests".
One of the sources of Disease X could be the deliberate utilization of mystery killer disease as a weapon.
Tom Frieden, a former leading disease control director in the USA, said: "We don't know where the next threat will come from".
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.
Marion Koopmans, WHO adviser, said, "The intensity of animal and human contact is becoming much greater as the world develops".