It is a theory that seems to be spreading as quickly as the virus itself: The new coronavirus is infecting relatively few in California because the state suffered a silent outbreak as far back as the fall, well before official reports indicate it hit the U.S. As a result, the theory goes, many residents are now resistant to the disease.

The hypothesis, sparked by an article written for the conservative National Review by a military historian affiliated with Stanford University, went viral in recent weeks. Questions around...