The World Health Organization added "Disease X" to its growing list of potential global disease threats that lack an effective drug or vaccine, according to CNN.
Here are four things to know.
1. The list, which was published in a strategy and preparedness plan known as the 2018 R&D Blueprint, outlines viruses the WHO believes could spark a worldwide epidemic. The blueprint helps prioritize research and development, surveillance and diagnostics.
2. Disease X doesn't exist. It is simply a placeholder for an "unexpected" disease and "represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease," according to the WHO.
3. The WHO believes there is a high probability the next big epidemic will come from a completely unexpected source, a pathogen that has yet to reveal itself.
"As experience has taught us more often than not the thing that is [going to] hit us is something that we did not anticipate," Anthony Fauci, MD, director of the U.S. National Institute of Allergy and Infectious Diseases, told CNN. "Just the way we didn't anticipate Zika, we didn't think there would be an Ebola that would hit cities."
4. Along with Disease X, the WHO named seven other global disease threats that lack a drug or vaccine, including: Ebola, Zika, Marburg virus disease, Crimean-Congo hemorrhagic fever, Middle East respiratory syndrome coronavirus, Severe Acute Respiratory Syndrome and Nipah virus.