How does a “new” disease suddenly appear and spread in such a short amount of time?

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It’s not like Plague Inc where you can just “create” a new disease, it had to have come from somewhere. How can a disease we haven’t seen before pop up seemingly overnight and spread worldwide in just a few months? Is it a sudden mutation of an existing disease? If so, what could cause such a quick transformation from something we know to something we haven’t seen before and are completely unprepared to fight?

In: Biology