The trifecta of fatigue, winter weather and holiday travel that has led to more coronavirus transmission across the country hit here, too — and became the match that lit the tinderbox.

The popular image of L.A. — hillside mansions, urban sprawl and drivers cocooned in their cars — belies the gritty reality. L.A. County, home to more than 10 million people, suffers from high rates of poverty and homelessness, huge numbers of essential workers and some of the densest neighborhoods in the nation.

“There is no city as large and complex as L.A. The closest might be New York. And we saw what happened in New York,” said Dr. Kirsten Bibbins-Domingo, an epidemiologist at UC San Francisco.

Experts also pin L.A.’s problems on rules that can appear inconsistent or arbitrary, as well as a confusing patchwork of policies across Southern California. Additionally, the county is investigating whether a more contagious strain of the virus, circulating in the United Kingdom, could be partly to blame.