Disease X" refers to a hypothetical, unknown pathogen that could potentially cause a future epidemic or pandemic. The term was coined by the World Health Organization (WHO) as part of their "Blueprint list of priority diseases" to highlight the need for research and preparedness against emerging infectious diseases that could pose a significant public health threat. The concept emphasizes the importance of being ready for unexpected outbreaks caused by novel or re-emerging pathogens that are not yet known or identified. It serves as a reminder of the need for flexible and adaptive approaches in global health strategies to quickly and effectively respond to new health threats.