What is disease X?
For the first time, the World Health Organization has included “disease X” in its annual list of priority infectious diseases. “A serious international epidemic could be caused by a pathogen currently unknown to cause human disease,” WHO says.1 Disease X might represent a new pathogen or the re-emergence of an old disease, like the last Ebola outbreak. Experts welcomed WHO’s inclusion of an unknown disease. “We cannot predict all emerging disease problems, in contrast to what the list of priority diseases suggests,” Marion Koopmans, head of the viroscience department of the Erasmus MC Rotterdam, told The BMJ. “Look, for instance, at the last flu pandemic in 2009—that arose from a swine flu influenza virus, most likely in the Americas, whereas the world was looking at Asia and avian influenza viruses as the most likely pandemic threat.”