Actually, disease X dates back to 2018, and was mentioned by WHO:

https://web.archive.org/web/20200301083134/http://origin.who.int/blueprint/priority-diseases/en/

The second annual review occurred 6-7 February, 2018. Experts consider that given their potential to cause a public health emergency and the absence of efficacious drugs and/or vaccines, there is an urgent need for accelerated research and development for*:

• Crimean-Congo haemorrhagic fever (CCHF)
• Ebola virus disease and Marburg virus disease
• Lassa fever
• Middle East respiratory syndrome coronavirus (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS)
• Nipah and henipaviral diseases
• Rift Valley fever (RVF)
• Zika
• Disease X

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.