"Disease X" refers to the possibility of an unknown pathogen causing a serious international pandemic. It can be triggered at any time and from multiple sources, potentially claiming millions of lives.
The concept was proposed to avoid or reduce as much as possible new or mutated pathogens such as The harm of the new coronavirus to humans. There are about 1.67 million unknown viruses that need to be discovered, of which 631,000 to 827,000 species may infect humans.