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A drug-resistant form of a bug that causes traveler’s diarrhea is causing outbreaks in the United States — and it’s got federal officials worried.

The medical name is Shigella sonnei, but it’s one of the germs that causes shigellosis — otherwise known as “Delhi belly” or “Montezuma’s revenge.” Many international travelers are sadly familiar with the stomach cramps, diarrhea and nausea that can ruin a vacation or business trip.

If the infection is caused by Shigella bacteria, it was once easy to treat with antibiotics. What has the Centers for Disease Control and Prevention worried is evidence that a mutant form is now not only being carried into the U.S. by travelers, but it’s spreading once it’s here.

“Shigella sonnei bacteria resistant to the antibiotic ciprofloxacin sickened 243 people in 32 states and Puerto Rico between May 2014 and February 2015,” the CDC said in a statement.

“Research by the CDC found that the drug-resistant illness was being repeatedly introduced as ill travelers returned and was then infecting other people in a series of outbreaks around the country.”

"The potential for more – and larger – outbreaks is a real concern."

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CDC epidemiologists and local health officials tested cases in Massachusetts, California and Pennsylvania and found that nearly 90 percent of the patients were infected with bacteria resistant to ciprofloxacin (Cipro), which usually is the first drug tried against shigellosis. Before last year, just 2 percent of cases were Cipro-resistant.

That’s bad news because most Shigella in the U.S. is already resistant to the antibiotics ampicillin and trimethoprim/sulfamethoxazole. It leaves fewer choices for treating the infections.

Shigella causes about 500,000 cases of diarrhea in the U.S. every year, CDC says. Most cases go away without any need for treatment. But it’s important to have the option, especially for severe cases and people whose immune systems are compromised, like HIV or cancer patients.

The CDC and the White House have been warning loudly about the rise of antibiotic-resistant superbugs. This adds another one to the mix.

“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,” CDC director Dr. Tom Frieden said in a statement.

“Shigellosis can spread very quickly in groups like children in childcare facilities, homeless people and gay and bisexual men."

Like most stomach bugs, Shigella is spread when people get it on their hands and then touch their mouths or noses. It’s one of the main reasons people are urged to wash their hands when using restrooms and especially when handling food.

“Shigellosis can spread very quickly in groups like children in childcare facilities, homeless people and gay and bisexual men, as occurred in these outbreaks,” CDC said.

“The increase in drug-resistant Shigella makes it even more critical to prevent shigellosis from spreading,” said Dr. Anna Bowen, a medical officer in CDC’s Waterborne Diseases Prevention Branch who led the study. “Washing your hands with soap and water is important for everyone. Also, international travelers can protect themselves by choosing hot foods and drinking only from sealed containers.”

About half of patients whose details were known had links with international travel, mostly to the Dominican Republic and India. In a San Francisco outbreak that sickened 95 people, half the victims were either homeless or living in single-room occupancy hotels.