The illness - now given the name Disease X - has the potential to spread quickly, and any cures or countermeasures are now severely lacking.
But the mystery threat has not been placed on the list to scare the public.
"Disease X" is not exactly new, as this killer pathogen is a so-called "known unknown" that could be created by biological mutations, like what happened with Spanish Flu or HIV, The New York Post reports.
Highlighted in the 2018 annual review of the Blueprint list of priority diseases, WHO said it developed a tool to determine which diseases and pathogens pose a public health risk due to their epidemic potential.
The review, which was released overnight, lists the disease alongside Zika, Ebola, SARS and Lassa fever as the viruses with the potential to cause a public health emergency.
According to experts, Disease X, in all probabilities will be man-made.
The World Health Organisation (WHO) has revealed details about "Disease X" in its second annual review of priority diseases.
Not a great deal is known about the disease yet, although World Health Organization warns that Disease X needed to be included on the list because of its unpredictable nature.
"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.
Disease X could spring up from a lot of different sources and infect us via innumerable vectors, Mr. Rottingen says, although zoonotic transmission (an animal virus evolving to infect humans) is the most likely.
"As the ecosystem and human habitats change there is always the risk of disease jumping from animals to humans", Rottingen said. "This makes it more likely new diseases will emerge, but also modern travel and trade make it much more likely they will spread", World Health Organization adviser Marion Koopmans explained to The Telegraph.
"It's a natural process and it is vital that we are aware and prepare". This is why is it more probable that new diseases will rise, yet, current travel and trade influence it substantially, so it means that they are even more likely to spread.