The World Health Organization is preparing for an unknown pandemic it’s calling “Disease X,” according to its annual list of priority diseases. The WHO says “Disease X” is a yet-to-be-determined infectious disease that may appear suddenly and spread quickly through humans, potentially through the respiratory system.

Additional diseases on the WHO list include MERS (middle east respiratory syndrome), SARS (sudden acute respiratory syndrome), and:

  • Crimean-Congo hemorrhagic fever;
  • Ebola virus disease and Marburg virus disease;
  • Lassa fever;
  • Nipah and henipaviral diseases;
  • Rift Valley fever;
  • Zika.

“Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease, and so the R&D Blueprint explicitly seeks to enable cross-cutting R&D preparedness that is also relevant for an unknown ‘Disease X’ as far as possible,” WHO said in a statement.