A dire lack of baby formula in the United States in the past few weeks has been blamed on production deficiencies such as the small number of manufacturers and an inflexible supply chain. But Christina Jewett, an investigative reporter at The Times, has traced it back further, to deadly bacteria whose detection set off a chain of events that ultimately led to the shortage. Guest: Christina Jewett, an investigative reporter who covers the Food and Drug Administration for The New York Times.