Scenario four was the most intricate. It imagined that some team of scientists had performed “passaging” experiments with a SARS-like coronavirus—that is, intentionally infecting a series of laboratory animals, each animal given the virus in the form that emerged from a previous animal, and thereby inviting the virus to adapt better and better to those animals as it went. Or the virus might have been similarly passaged not in live animals but in cultured cells, dish by dish, using laboratory-captive strains of once human (or at least once primate) cells.
