Yuma County, Ariz., which produces the lettuce, broccoli and other leafy greens that Americans consume during the cold months, is known as “America’s salad bowl.” Now it has become a winter hothouse for Covid-19.

Over the course of the pandemic, the Yuma area has identified coronavirus cases at a higher rate than any other region in the United States. One out of every six residents has contracted the virus.

Each winter, the county’s population swells by 100,000 people, to more than 300,000, as field workers descend on the farms and snowbirds from the Midwest pull into R.V. parks. This seasonal ritual creates jobs and increases local spending and tax revenue. But this year, the influx has turned deadly.

Even as coronavirus cases are starting to flatten across the country, the virus is still raging in many border communities. Three of the six metro areas with the highest rates of known cases since the outbreak began are small cities straddling Mexico: Yuma; Eagle Pass, Texas; and El Centro, Calif. In Laredo, Texas, cases are increasing at more than three times the rate being reported in hard-hit Los Angeles and Phoenix.

Seasonal migration, the daily flow of people back and forth and lax measures to contain the virus’s spread have created a combustible constellation. Arizona has among the highest increases in newly reported deaths of any state over the past two weeks — and it is not clear when this troubling trend will abate.

“Some families have buried multiple relatives,” said the Rev. Emilio Chapa, the pastor of St. Francis of Assisi Catholic Church in central Yuma. “It’s a dire situation.”