“Disease X” doesn’t actually exist. The WHO wasn’t warning against a specific pathogen; it was reminding public health and medicine how important it is to be ready for any new illness—especially since existing tests and might not be precise enough to detect a new disease that has epidemic potential, and treatments may not be potent enough to stop it.
In the four years that the list has been published, no emerging infection has been serious enough to rise to the level of Disease X: a pathogen that could sweep the world before science catches up. But a new syndrome, acute flaccid myelitis, or AFM, is providing the first proof of the need for that warning. As perplexing to diagnose as it is to treat, AFM is demonstrating how difficult it can be to understand and predict any new disease. And the challenge of tracking an uncommon illness is giving us a glimpse of how our surveillance systems will struggle to counter the world-spanning epidemic that Disease X may turn out to be.
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