According to CNN, In the most recent global plan for health emergencies the World Health Organization included something called "Disease X".

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Every year the World Health Organization releases what they call the "R&D Blueprint" which stands for research and development. According to CNN, this became a thing after the Ebola epidemic in 2015. This is how they prepare for unexpected diseases and illnesses like Ebola and Zika. They create a plan that will allow them to do accelerated research and an action plan to combat the diseases. You know, to keep people safe.

In the 2018 R&D Blueprint, there is a mention of "Disease X". Which sounds super scary. Like something out of a cinematic thriller. However this is what they are calling the next unknown "pathogen" that could create the next international epidemic.
The name might sound scary, but it's just a filler word for the next big disease to come after humanity with "x" standing for unknown. Should you feel concerned about this? The answer is no. At least the World Health Organization is preparing instead of waiting on stand by.