The World Health Organization has a list of "priority viruses" that could cause the next deadly pandemic.
Most of these diseases are already known, Ebola, SARS and Zika have been "silent" in recent years, but on the WHO list is a virus with the terrible name "Disease X". The term "Disease X" is used by the WHO as a surrogate to describe a disease that is unknown to medical science.
And like a disease whether it's a virus, bacteria, or something else there is most likely no vaccine or treatment available.
"This is a scenario for which we must prepare. This is disease X", said the prominent American epidemiologist Dr. Richard Hatchet.
Scenarios for the origin of "disease X" or "zombie" viruses
Epidemiologists believe that the next disease X will be zoonotic, meaning it will come from wild or domestic animals and then spread and infect the population. Ebola, HIV/AIDS and COVID-19 were zoonotic epidemics.
"The release of such viruses, either through laboratory accidents or as an act of bioterrorism, can lead to a fatal disease X and has been described as a global risk," the scientists say.
Another possible source could be "zombie" viruses that have been trapped for centuries but are being released by a warming climate.