World Health Organization warned that a number of other potentially-deadly diseases that didn't make the list should still be considered a high priority.
The report comes after the committee convened in Geneva at the World Health Organization headquarters 6th and 7th of February.
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.
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. Each of these outbreaks have been hard to combat.
This is the first time that Disease X has made the list, during a convention of experts on viruses, bacteria and infectious diseases. The report comes this February warning the public of the major threats worldwide.
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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".
Disease X represents the knowledge that a serious worldwide epidemic could be caused by a pathogen now unknown.
"It may seem unusual to be adding an 'X, ' but the point is make sure we prepare and plan flexibly in terms of vaccines and diagnostic tests", Rottingen told the newspaper.
HIV is believed to have jumped from chimpanzees to humans and has killed 35 million people since the 1980s. Around 70 percent of the recently known diseases that affect populations are known to be zoonoses. These include Severe Fever with Thrombocytopenia Syndrome (SFTS), emergent non-polio enteroviruses including EV71, D68 and Chikungunya.
The World Health Organisation (WHO) has revealed details about "Disease X" in its second annual review of priority diseases. These may however soon be included in the list, the agency said.