The WHO held it's 2018 annual review of the R&D Blueprint list of priority diseases on February 6-7. The revised list includes the undefined "Disease X." The WHO says that "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."
http://www.who.int/blueprint/priority-diseases/en/http://archive.is/O7UzX"Disease X" is a reference to the X (female) chromosome, sending a message the the WHO is concerned about the Femin Virus and is seeking further information about the pathogen.
https://ghr.nlm.nih.gov/chromosome/Xhttp://archive.is/F2zEthttp://en.wikipedia.org/wiki/X_chromosomehttp://archive.is/sb4t9https://www.sciencealert.com/here-s-why-is-the-x-chromosome-so-oddhttp://archive.is/KihshIn their meeting report, the WHO mentioned several aspects of the pathogen that they would focus on:
-Human transmission
-Severity or case fatality rate
-The human/animal interface
-Other factors (including the pathogens geographic range, shared epidemiological and/or genotypic characteristics with pathogens that pose an epidemic threat, the absence of robust protective immunity, a high risk of occupational exposure, or connections with biological weapons programmes)
-Public health context of the affected area
-Potential societal impacts
-Evolutionary potential
https://via.hypothes.is/http://www.who.int/emergencies/diseases/2018prioritization-report.pdf 