There are many possible theories as to why California was hit so hard starting in the late fall, including the introduction of more contagious strains of the virus, dry weather that made transmission easier and a higher percentage of the population being vulnerable to the disease because relatively few Californians had been infected up to that point.

But most experts point to changes in behavior: people beginning to abandon staying home, social distancing while out and other precautions that experts say curb transmission of the coronavirus.

In the fall, masking dipped in California while social distancing fell to the lowest levels since the pandemic began, according to one analysis. Meanwhile, the numbers of Californians attending gatherings with 10 or more people reached the highest level since before March, according to a USC survey.

And when a coronavirus wave started building in late October, Californians didn’t cut down on their risky activities as quickly as they had earlier in the pandemic.