To alleviate the pressure, hospitals in nearby San Diego and Riverside counties began accepting transfers in April. But the intensifying crisis prompted California last week to activate an extraordinary response, enlisting hospitals as far north as Santa Barbara, San Francisco and Sacramento to accept patients from this remote southeastern corner of the state.

Last week, a patient was being transferred from the hospital in El Centro every two to three hours, compared to 17 in an entire month before the Covid-19 pandemic, Ms. Cruz said on a recent morning as a helicopter prepared to airlift a patient and five ambulances dropped off patients near a trio of tents erected outside the hospital to triage new arrivals.

The swelling numbers of Covid-19 patients entering the United States from Mexico comes as many parts of California have pushed down their infection rates, enabling many counties to lift stay-at-home restrictions and reopen businesses.

“We worked hard to flatten the curve in California,” said Carmela Coyle, president of the California Hospital Association, who issued an appeal to hospital systems across the state for help. “Now we have a surge in the Imperial Valley because the situation is so severe in Mexicali.”