Disease X
There is a newly discovered disease that can cause a widespread infection across the globe. As for where Disease X could come from, nobody knows for sure, but there a multitude of possible sources, including existing viruses that demonstrate new virulence and symptoms (such as Zika virus), engineered viruses escaped from laboratories or used as bioweapons, and zoonotic infections transferred from animals to humans.
WHO or World Health Organisation says that we need to be prepared for it, which is why the mystery malady is now on the agency's R&D Blueprint of priority diseases.
It is scary that this might really happen is it because of over population and the effects of pollution?