Experts say a variety of factors combined to wipe out the past efforts, which for much of the year held the virus to manageable levels. Cramped housing, travel and Thanksgiving gatherings contributed to the spread, along with the public’s fatigue amid regulations that closed many schools and businesses and encouraged — or required — an isolated lifestyle.

Another factor could be a more contagious variant of the virus detected in Southern California, although it’s not clear yet how widespread that may be…

Hospitalizations statewide have gone up more than eightfold in two months and nearly tenfold in Los Angeles County. On Thursday, the total number of California deaths surpassed 25,000, joining only New York and Texas at that milestone.

“Most heartbreaking is that if we had done a better job of reducing transmission of the virus, many of these deaths would not have happened,” said Barbara Ferrer, the county’s public health director, who has pleaded with people not to get together and worsen the spread.