Let’s get it right from the start: there is no such thing as Disease X. The concept of «disease X» was introduced by World Health Organization as a placeholder for predicted rise of «known unknown» epidemic of unclear origin. The purpose of the introduction of «Disease X» is to leave some space open for a new epidemic that might be caused by an unknown pathogen. It is an official acknowledgement that medics are not in control of new and potentially highly aggressive stains of known viruses and bacteria and the emerging ones. WHO adopted the «Disease X» in 2018.
This explanation is quite logical, however sensations followed. Just take a look at the title: «The mystery killer keeping scientists awake at night». Of course it is a cynical clickbait and the article actually confirms it. Alarmed communities even prepared to fight with the mysterious epidemic. Some people began to think of unimaginable horrors of government conspiracy.
Well, there is a conspiracy after all, although not of the evil kind. The decision to introduce Disease X as a epidemic agent will allow to allocate some additional funds in case of the real «known unknown» will happen. As it was with Zika, Ebola, Bird Flu or HIV. The concept of Disease X just rises preparedness of medical research and development teams and facilities.