Before boarding a flight from Orlando to Los Angeles, Isaias Hernandez filled out a health checklist provided by United Airlines, asserting that he had not been diagnosed with COVID-19 and had not shown any of the disease’s symptoms in the previous two weeks.

But during the flight, the 69-year-old Angeleno collapsed. Three passengers gave him CPR for nearly an hour in the aisle of the plane, and the flight was diverted to Louisiana, where Hernandez was pronounced dead. The coroner’s report listed the cause as “acute respiratory failure, COVID-19.”

The Dec. 14 incident illustrates the deficiencies in the systems that are meant to prevent people from bringing the coronavirus aboard commercial flights and potentially spreading it to the people packed in around them. And it happened as holiday air travel ramped up. In the days surrounding Christmas, more than a million passengers boarded planes almost daily, reaching 1.3 million last Sunday — the most since March.