An outbreak of a stomach bug that was tied to swimming in one Oregon lake highlights a perhaps less obvious hazard of summer fun.

Last July, 70 people became ill with norovirus — a very contagious virus that causes vomiting and diarrhea — after visiting a lake near Portland, according to a new report from the Centers for Disease Control and Prevention. More than half of those who got sick were kids ages 4 to 10.

The researchers determined that the lake was likely the source of the infection after they found that people who swam in the lake were about two times more likely to develop vomiting or diarrhea, compared to people who visited the park but didn't swim, the report said. People typically developed symptoms about a day after they were exposed to the virus.

The outbreak is important because people usually think of norovirus as a bug that people catch on cruise ships or in swimming pools, whereas the new report highlights the need "to inform the public about the risks associated with swimming in untreated recreational water venues," the researchers said.

Children, in particular, are at risk for norovirus infection in lakes or swimming pools, because they are more likely to swallow water, said Michael Beach, associate director for healthy water at the CDC. "Keeping germs out of the water in the first place is key to keeping everyone healthy and helping to keep the places we swim open all summer," Beach said. [10 Health Woes Summer Can Bring]

Researchers said they don't know how the lake in Oregon became contaminated with norovirus, but suspect that a swimmer vomited or had diarrhea in the lake, which started the outbreak. The lake was closed for 10 days to prevent other swimmers from getting sick.

But because there are no steps to take to decontaminate a lake after a norovirus outbreak, health officials found it challenging to determine exactly when it would be safe to reopen the body of water. They decided to wait much longer than the virus's incubation period before allowing people to swim again.

"Public health agencies could benefit from the development of evidence-based criteria to determine when to reopen untreated recreational water venues associated with outbreaks," the researchers said.

The CDC recommends the following tips for healthy swimming:

  • Don't swim if you have symptoms of diarrhea, or if you've been vomiting.
  • Don't swallow swimming water.
  • Take a pre-swim shower.
  • Don't pee or poop in the pool or lake.
  • Parents should take their children on bathroom breaks every hour. For very young children, parents should check diapers frequently, and keep dirty diapers away from the water.

The report is published this week in the CDC's Morbidity and Mortality Weekly Report.

Copyright 2015 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.