According to a new report released by the Centers for Disease Control and Prevention (CDC), international travelers are transporting a multidrug-resistant intestinal illness to the United States and spreading it among Americans.

Between May 2014 and February 2015, Shigella sonnei bacteria, which is resistant to the antibiotic ciprofloxacin, sickened 243 people in 32 states and Puerto Rico.

The CDC and public health partners have investigated recent outbreaks of shigellosis in Massachusetts, California and Pennsylvania. They found that nearly 90 percent of the cases tested were resistant to ciprofloxacin. The antibiotic is the first choice for treating shigellosis, which can spread quickly in groups like children in childcare facilities, homeless people, and gay and bisexual men, all of which occurred in these outbreaks.

“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. Thomas Frieden, M.P.H., said in a news release. “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.”

Shigella in the U.S. has already been found to be resistant to other antibiotics— ampicillin and trimethoprim/sulfamethoxazolex¬— as its resistance to ciprofloxacin is increasing. The drug is prescribed to people traveling internationally, in case of diarrhea, but more study is needed to determine what, if any, role the use of antibiotics during travel may have in increasing the risk of antibiotic-resistant diarrhea infections.

“The increase in drug-resistant Shigella makes it even more critical to prevent shigellosis from spreading,” Dr. Anna Bowen, M.P.H., a medical officer in CDC’s Waterborne Diseases Prevention Branch and lead author of the study, said in the news release. “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.”

According to the CDC, Shigella causes an estimated 500,000 cases of diarrhea in the U.S. every year. The illness spreads through contaminated food and recreational water, and  can cause watery or bloody diarrhea, abdominal pain, fever and malaise.