Jonah Preston, a five-year-old boy with a history of asthma, was brought to triage by his mother early one morning with a chief complaint of “difficulty breathing.” (This case is a composite based on our experience.) Ms. Preston reported that her son had recently been sneezing and had a runny nose. Over night, he’d begun to cough, finally waking her at 5 am complaining that he couldn’t breathe....