Disease X is a placeholder name that was adopted by the World Health Organization (WHO) in February 2018 on their shortlist of blueprint priority diseases to represent a hypothetical, unknown pathogen that could cause a future epidemic. The WHO adopted the placeholder name to ensure that their planning was sufficiently flexible to adapt to an unknown pathogen (e.g., broader vaccines and manufacturing facilities). Director of the US National Institute of Allergy and Infectious Diseases Anthony Fauci stated that the concept of Disease X would encourage WHO projects to focus their research efforts on entire classes of viruses (e.g., flaviviruses), instead of just individual strains (e.g., zika virus), thus improving WHO capability to respond to unforeseen strains. In 2020, experts, including some of the WHO's own expert advisors, speculated that COVID-19, caused by SARS-CoV-2 virus strain, met the requirements to be the first Disease X.
