If you’re planning to travel abroad this summer, be wary of Shigella. The drug-resistant pathogen, a form of food poisoning, is spreading quickly in the U.S. and has been linked to international travel, said the Centers for Disease Control and Prevention in a study published on Thursday in the Morbidity and Mortality Weekly Report.

The bacterial illness is extremely contagious, spreading through contaminated water and food. Once contracted, the bacteria spreads to the intestines, inducing vomiting and mucus or bloody diarrhea. Unfortunately, it’s resistant to the most potent available treatment option, Ciprofloxacin, which typically assuages the pain of similar bacterial illnesses.

Experts say that if the resistance continues, Shigella may become virtually untreatable with oral antibiotics. If this happens, antibiotics likely would have to be administered through IV.

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Several outbreaks of Shigella have been reported in recent years, and more than 157 people fell ill during the time of the study, from May 2014 to February 2015. The bacterial strain was detected in 32 states, with sizable numbers of cases in Massachusetts and California.

In the study, researchers discovered that about half of the cases of the foodborne illness had been linked to travel, particularly to Morocco, India and the Dominican Republic. Yet several people who contracted the pathogen hadn’t traveled internationally, leading researchers to believe it may be domestically rooted as well. Authorities caution that the illness has continued to spread since the report was filed.

Researchers advise people to wash their hands often and to be aware of any bowel changes while traveling.

Shigellosis, a full-on infection caused by the bacteria Shigella, afflicts more than 80 million people a year, according to statistics by the CDC, and about 600,000 people a year die from the bacterial disorder.