There have been multiple instances of novel pathogen emergence that have affected the health and security of the global community. To highlight that these novel pathogens presented a clear danger to public health, the WHO included "Disease X" on their list of priority pathogens in 2018. Indeed, since the emergence of SARS-CoV-2, Disease X has been pointed to as the looming threat of "the next big thing." However, developing surveillance and preparedness plans with Disease X as the linchpin is too narrow and ignores a large swath of potential threats from already identified, often neglected diseases. We propose instead the idea of "Disease f(x)" as a preferred call to arms with which to prioritize research and programmatic development. The common mathematical notation f(x) represents the knowledge that outbreaks are a function of many variables that define the transmission trajectory of that pathogen. Disease f(x) exploits commonalities across pathogen groupings while recognizing that emergences and outbreaks are fluid and that responses need to be agile and progressively tailored to specific pathogens with cultural and regional context. Adoption of this mindset across sectors, including biotechnology, disaster management, and epidemiology, will allow us to develop more efficient and effective responses to address the next major infectious threat.