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 Organization (WHO) has published a list of priority diseases for 2018 and encourages researchers to guide their efforts towards these diseases for which there is either no treatment or insufficient treatment.
World health experts are warning of the existence of an illness, known only as "Disease X", which has the potential to trigger a serious worldwide epidemic.
Every year, the World Health Organization publishes an annual Blueprint review that lists diseases and pathogens to prioritise for research and development.More news: OJ Simpson Was Once Asked to Appear on 'The Simpsons'
The rest of the deceases which are listed in this category are known fatal pandemics as Ebola, Lassa Fever, CCHF hemorrhagic fever, Nipah / Henipaviral, MESS, SARS and Zika.
The WHO hopes to improve disease surveillance and the capacity of local health systems, which would help to detect an outbreak quickly.
Disease X represents the knowledge that a serious global epidemic could be caused by a pathogen now unknown.
"It may seem odd to be adding an "X" but the point is make sure we prepare and plan flexibly in terms of vaccines and diagnostic tests".More news: Schools, colleges closed in J&K after three militant killed in encounter
This year, for the first time, the World Health Organization 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.
"As the ecosystem and human habitats change there is always the risk of disease jumping from animals to humans", Rottingen said.
"It is probably the greatest risk".
Tom Frieden, a former leading disease control director in the United States, said: "We don't know where the next threat will come from".More news: Scott Baio's Wife Renee Diagnosed with Brain Disease
Killing Disease X before it can spread is particularly key in the modern world where travel and trade make it more likely to move around the globe than pandemics of the past.