It’s a hypothetical reality. FTA: “Disease X is a placeholder name adopted by the World Health Organization (WHO) in February 2018 on its shortlist of blueprint

It’s a hypothetical reality. FTA: “Disease X is a placeholder name adopted by the World Health Organization (WHO) in February 2018 on its shortlist of blueprint priority diseases. It represents a hypothetical, unknown pathogen that could cause a future epidemic or pandemic” See also https://en.wikipedia.org/wiki/Disease_X https://en.wikipedia.org/wiki/Disease_X: Since 2015, the shortlist of less than 10 EIDs has been updated annually and has consistently included widely known names such as Ebola, Zika and SARS (e.g., cause of large-scale infections), and more geographically specific names such as Lassa fever, Marburg virus, Rift Valley fever, and Nipah virus. “In February 2018, after the "2018 R&D Blueprint" meeting in Geneva, the WHO added Disease X to the shortlist as a placeholder for a "knowable unknown" pathogen. The Disease X placeholder acknowledged the potential for a future epidemic that could be caused by an unknown pathogen, and by its inclusion, challenged the WHO to ensure their planning and capabilities were flexible enough to adapt to such an event.”