The coronavirus has spread to over 35 countries and infected over 80,000 people worldwide, leading one health expert to believe it might be the theoretical unknown disease called "Disease X."

So what is Disease X?

"Disease X" is a term created by the World Health Organization for an unknown disease that could eventually spark a pandemic, according to a health expert at the organization.

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It was used as a placeholder on their list of illnesses that have the potential to reach international epidemic levels, which were identified from a blueprint created to help officials prepare for emerging diseases after the Ebola crisis in West Africa.

“Whether it will be contained or not, this outbreak is rapidly becoming the first true pandemic challenge that fits the disease X category,” Marion Koopmans, a member of WHO's emergency committee told Fox News.

Some of those infected by the coronavirus have had no link to China.

“There is a number of diseases on that list that we know, but also ‘Disease X,'" Koopmans added. “That is meant to alert the world to think about how to prepare for these diseases.”

Diseases prioritized on the list include Crimean-Congo hemorrhagic fever (CCHF), Ebola and Marburg virus disease, Lassa Fever, MERS, SARS, Nipah and henipaviral diseases, Rift Valley fever and Zika.

Coronavirus, or COVID-19 as defined by the WHO, "fits the Disease X category," Koopmans wrote in the journal Cell last week.

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“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," according to its official definition on the page.

One of the most difficult parts of stopping the virus is its 14-day incubation period, which means symptoms might not be visible during that time. Because of that officials are usually playing catch up.

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California recorded its first case of the virus on Wednesday from a person who reportedly didn't travel abroad or come in contact with another known infected person.

All other 59 cases recorded in the U.S. had traveled outside the country.

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“Pandemic simply means there is global widespread circulation,” Koopmans told Fox News. “The blueprint diseases can also be a disease with regional major impact.”

Fox News' Alexandria Hein contributed to this report