[rovolo]

The WSJ article is behind a paywall so I can't double check this, but you can have a 99% accurate test with a 20% misdiagnosis rate. The difference is that 99% is per-person, and 20% is per-positive-result. A rare condition will have more false-positives relative to true-positives than the test accuracy would indicate.

It's a bit easier to understand if you think of the extreme case: if the disease is completely eliminated than all positives are false-positives.