“In a research paper for the Institute for Law and Economics, law professors Jonathan Klick and Joshua Wright crunched state and federal data on emergency room admissions...”
No harm, no foul; most San Franciscans are long since used to hospital stays for communicable infections.
That seems to be their point. Paraphrase: "Given what we all know about the (ahem) personal habits of San Franciscans, a little extra e.coli due to changing grocery bag usage wouldn't show up in the stats. Maybe they should run some numbers in communities where everyone doesn't, well, you know ..."