Some international travelers are bringing back and spreading a bacteria that’s resistant to the drugs used to treat it, the U.S. Centers for Disease Control and Prevention (CDC) reported on Thursday in an investigation published in the CDC’s Morbidity and Mortality Weekly Report.

The bug, called shigella sonnei, causes an estimated 500,000 cases of diarrhea in the U.S. each year, and the CDC reports that between May 2014 and February 2015, a drug-resistant strain infected 243 people in 32 states and Puerto Rico. When investigating clusters of shigellosis—the infection caused by shigella—in Massachusetts, California and Pennsylvania, the CDC discovered that 90% of the cases were resistant to an antibiotic called ciprofloxacin (Cipro), which is the drug of choice when treating shigellosis.

Most shigella strains are already resistant to other drugs used to treat it, and public health experts have noted that a Cipro-resistant strain is spreading worldwide. The bacteria can spread very quickly and are commonly discovered in places like childcare centers and nursing homes, according to the report.

“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,” said CDC director Dr. Tom Frieden in a statement. Frieden added that the recent outbreaks reveal a “troubling trend” of infections in the U.S.

The CDC was first alerted to a new strain of shigella in December 2014 and discovered in the lab that it was resistant to Cipro. The CDC then investigated several large clusters of shigella infections across the United States. Nearly 100 cases were from an outbreak among homeless people in San Francisco, and others were related to international travel. Infection can be common among people who travel to developing countries.

The CDC says international travelers should wash their hands “meticulously” and be cautious about food and water consumption. Travelers can download the CDC’s app, “Can I Eat This?” as a guide for what’s safe to eat while traveling.

Antibiotic resistance is becoming a growing global problem; in September 2014, President Obama signed an executive order to create a task force to tackle the issue of antibiotic-resistant bacteria.

Get our Health Newsletter. Sign up to receive the latest health and science news, plus answers to wellness questions and expert tips. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.