Every year, the World Health Organization convenes scientists and health workers from around the world to identify diseases with epidemic or pandemic potentials.
These diseases could kill millions but currently, do not have enough countermeasures available.
In the previous years, leading bacteriologists, virologists, and infectious disease experts identified known killers such as the Lassa fever and Ebola, the hemorrhagic disease that killed over 11,000 people in West Africa between the years 2013 and 2016.
This year, however, a still unknown pathogen has been added to the list. The world's leading health experts have identified Disease X as a global threat.
WHO said that Disease X could be caused by a pathogen that is not yet identified to cause human disease. Disease X is something that may go unnoticed until it is too late when it has already spread fast and caught the world by surprise.
"Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease, and so the R&D Blueprint explicitly seeks to enable cross-cutting R&D preparedness that is also relevant for an unknown "Disease X" as far as possible," WHO said in a statement.
Experts said that Disease X may emerge from a variety of sources and may strike anytime. It could emerge from an accident or act of terror amid advances in gene-manipulating technologies that now make possible the creation of entirely new viruses.
While WHO focuses on yet to be identified pathogens in its description of Disease X, there is also the risk that existing diseases could evolve.
Tuberculosis, for instance, is a constantly evolving disease. In its most basic form, TB infection can be treated using simple antibiotic but the bacteria behind tuberculosis has evolved resistance to an antibiotic. A multidrug-resistant TB has so far been reported in 117 countries and affected about 490,000 people worldwide.
Disease X could also be sparked by diseases jumping from animals to humans and then spread to become an epidemic or pandemic.
There are currently more than 200 zoonotic diseases caused by viruses, parasites, bacteria, and fungi that are directly or indirectly transferred by animals to humans. These include MERS, Ebola, anthrax, bubonic plague, and mad cow disease.
"Scientists estimate that more than 6 out of every 10 known infectious diseases in people are spread from animals, and 3 out of every 4 new or emerging infectious diseases in people are spread from animals," the U.S. Centers for Disease Control and Prevention said.