The theoretical disease is ranked with Ebola, Lassa fever and the Zika virus, among others, as a research priority.
Earlier this year, the WHO listed Disease X and Zika as one of the top 10 greatest threats to global health in 2019.
The WHO said: “WHO’s R&D Blueprint identifies diseases and pathogens that have potential to cause a public health emergency but lack effective treatments and vaccines.
“This watchlist for priority research and development includes Ebola, several other hemorrhagic fevers, Zika, Nipah, Middle East respiratory syndrome coronavirus (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS) and Disease X, which represents the need to prepare for an unknown pathogen that could cause a serious epidemic.”