May 16, 2015 9:57 AM EDT

Flu season is over, but with the summer comes health concerns of a different pathogenic sort. An outbreak of the stomach bug norovirus last summer was linked to a popular lake destination in Oregon, found a new study released in the Center for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

After the weekend of July 12, 2014, the Multnomah County Health Department received word of 13 cases of acute gastrointestinal illness from people who’d visited Blue Lake Regional Park, a popular lake near Portland, Ore., the weekend prior. The investigation identified 70 likely cases of norovirus, which causes stomach flu and is most famously known for striking cruise ship passengers, from the weekend of July 11-13. About 15,400 people visited the park that weekend, and the lake was closed for 10 days to control the outbreak. People who went swimming in the lake were 2.3 times more likely to get sick than those who visited but didn’t go in the water.

Though the authors weren’t able to say for sure, the most likely cause of the outbreak was “a swimmer’s vomit or fecal incident in the lake,” the report reads. Lakes are especially vulnerable, the authors write, since they tend to attract small children and are not chemically treated.

This isn’t the first time the lake has caught a nasty virus. In 1991, Blue Lake was linked to an outbreak of E. coli and Shigella, and in 2004, it had an outbreak of norovirus that affected more than 100 people.

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