The Centers for Disease Control and Prevention reported this week that the bacteria Shigella sonnei has become resistant to ciprofloxacin, the antibiotic that used to shorten the illness it caused and lessen its severity. The agency identified 243 cases of shigellosis in 32 states and Puerto Rico. Testing 126 of them, officials determined that 109 cases were resistant to antibiotics. William Schaffner, professor of preventive medicine and health policy, is quoted.