'Disease X': The Mystery Malady That Could One Day Kill Millions

A new global threat dubbed "Disease X" has been added to the World Health Organization's (WHO) list of maladies that could cause a worldwide epidemic-even though it doesn't actually exist yet.

The mysterious malady is meant to represent a "known unknown" that could be created by biological mutation in the future, WHO said on its website.

The disease joined a list that includes the Ebola virus, Respiratory Syndrome (SARS,) Zika virus and Rift Valley fever (RVF), after an annual review by WHO.

'Disease X' represents the knowledge that a serious epidemic could be caused by a pathogen currently unknown to cause human disease World Health Organization

"Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease," WHO said on its website.

The disease could come about a number of ways, including as a result of biological warfare or as the result of the sudden spread of a virus similar to the "Spanish flu" which affected large parts of the world and is thought to have killed at least 40 million people in 1918-1919.

The organization also considered adding a number of other diseases to its list of global threats, including arenaviral hemorrhagic fevers, Chikungunya, a viral disease transmitted to humans by infected mosquitoes, and highly pathogenic coronaviral diseases.

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"These diseases pose major public health risks and further research and development is needed, including surveillance and diagnostics," WHO said. "They should be watched carefully and considered again at the next annual review."

WHO's Research and Development (R&D) Blueprint list seeks to identify diseases that pose a public health risk due to their potential for sparking an epidemic and for which there are no, or insufficient, countermeasures.

The organization initially released the list of prioritized diseases in December 2015.

It added Disease X to the list after an annual review carried out from Feb. 6-7, warning that there is an "urgent need" for accelerated research and development for the maladies on the list.