Seems like this was planned long time ago, as disease X dates back to 2018, where it 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.

They're so specific because they know what they designed the C19 jab to do.

Nitter link may or may not work:

https://nitter.rawbit.ninja/DougAMacgregor/status/1752397935394886039#m

X= depopulation + Black Horse

First they considered a whole new label illness (marburg) to account for all the blood and heart related vaxx side effects. But now I think you are right, it has expanded to a "new virus" X, that accounts for all the deadly responses to the vaxx

My bet is "jab reactions plus anything else they can cook up."