The ordinary laws no longer govern. Every day, new rules are being written to deal with the crisis. We are living under an emergency constitution invoked by Facebook, Google, and other major tech platforms. In normal times, these companies are loath to pass judgment about what’s true and what’s false. But lately they have been taking unusually bold steps to keep misinformation about COVID-19 from circulating. As a matter of public health, these moves are entirely prudent.