A nasty, antibiotic-resistant bug struck at least 243 people in the United States during the 10-month period ending in February, including six patients in Philadelphia and 12 elsewhere in Pennsylvania, federal and state health officials said Thursday.

Though they rarely die, people infected with Shigella bacteria can suffer bloody diarrhea and intense abdominal pain for up to a week.

The best defense is hand-washing, as the microbe is commonly spread when an infected person touches other people or prepares food for them, said Bennett Lorber, professor at the Temple University School of Medicine. Among those at increased risk are children and their caregivers in day-care centers, the sites of frequent diaper changes.

The microbe infects 500,000 people a year, but the 243 new cases, reported by the federal Centers for Disease Control and Prevention, are worrisome because they did not respond to the drug ciprofloxacin, which normally does the trick overnight. Sold as Cipro, the antibiotic is often given to international travelers in case they develop diarrhea.

Doctors have other antibiotics they can use against Shigella and the disease it causes, called shigellosis, but these drugs have to be given intravenously in a hospital, Lorber said.

"This is still a very small problem, but it's an alarming problem because it's resistant to what has been the most useful drug to treat it," the physician said.

Lorber, an infectious diseases specialist at Temple, added there likely had been more drug-resistant infections than the 243 that were confirmed through laboratory testing.

In 40 of the cases, patients got sick while traveling to the Dominican Republic, India, Haiti, and Morocco, among other destinations, then transmitted the bug to others once back in the U.S., the CDC said.

Most of the six Philadelphia infections were acquired here, and none was fatal, said Jeff Moran, a spokesman for the city Department of Public Health.

With 95 reported cases, San Francisco had the most patients, many of whom were homeless or lived in single-room occupancy hotels, the CDC said. Another cluster of 45 cases occurred in Massachusetts.

From May 2014 to February 2015, the drug-resistant infections were reported in 32 states overall, as well as Puerto Rico, the agency said.

Numerically speaking, the top source of U.S. food-borne infection is Salmonella, Lorber said. But Shigella is more insidious in a way, because it does not take much of the bacteria to get sick.

A person will get sick from ingesting 100,000 Salmonella organisms, he said. With Shigella, it only takes 100.