NEW YORK -- A drug-resistant strain of a nasty stomach bug made its way into the U.S. and spread, causing more than 200 illnesses since last May, health officials said Thursday.

Many cases were traced to people who had recently traveled to the Dominican Republic, India or other countries.

The illness is called shigellosis; it's caused by the shigella bacteria. The bug is extremely contagious and outbreaks are not unusual, but health officials are concerned because this strain is resistant to the antibiotic most commonly prescribed for adults.

"This is the first time we've documented this large an outbreak of antibiotic-resistant (shigella) linked to international travel," said Dr. Anna Bowen of the Centers for Disease Control and Prevention.

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Since last May, the imported superbug has sickened at least 243 people in the U.S., with large recent outbreaks in Massachusetts, Pennsylvania and California.

Shigella bacteria spread very easily through contaminated food or in pools and ponds. The bacteria can be spread by people who are not showing any symptoms of illness. Outbreaks also are common at daycare centers when staff members don't wash their hands well enough after changing diapers.

Symptoms of illness, including stomach pain, diarrhea and fever, normally develop one or two days after exposure. For most people, it's an unpleasant but temporary illness that ends within a week and can be helped with over-the-counter medicines like Pepto-Bismol or Imodium.

Sometimes doctors prescribe antibiotics: usually azithromycin for children and ciprofloxacin, sold as Cipro, for adults.

But over the past few years, health officials have been detecting shigella bugs resistant to azithromycin. Now the Cipro-resistant superbug has sickened people in 32 states and Puerto Rico.

"These outbreaks show a troubling trend," Dr. Tom Frieden, the director of the CDC, said in a statement. "Drug-resistant infections are harder to treat and because shigella spreads so easily between people, the potential for more - and larger - outbreaks is a real concern. We're moving quickly to implement a national strategy to curb antibiotic resistance because we can't take for granted that we'll always have the drugs we need to fight common infections."

Bowen also noted that simple precautions can help prevent illness: washing hands thoroughly with soap and water, and drinking only from sealed containers in countries where the water supply might be affected.