NORTH BEND, Wash. -- Some 1,600 residents who live in the Riverbend development east of North Bend are being told to boil their water before drinking it, after routine tests of the water supply showed contamination of e.coli bacteria.

The order affects the 533 homes who use the Riverbend Homeowners Association water system. Their development entrance is off SE 146th Street, and the advisory affects only the residents of that area.

Homeowner Association president Steve Smith flagged down everyone driving home to make sure they know to boil their water.

"We haven't had any outbreak for about 5 years now," Smith said.

It happens now and then. Water systems regularly take random samples for testing. This time, one of the samples came back positive for e.coli.

"So we immediately took 12 additional samples and those results are in the lab," said Roger Lillejord with the Riverbend Water System.

Lillejord said he hopes to get the results back from the health department over the weekend. In the meantime, he's going to door to door to tell people they can't use their tap water for drinking, cooking, making ice, brushing their teeth or washing dishes, without boiling it first. The health department says bring the water to a rolling boil for at least a minute, then let it cool.

"I have an 81-year-old grandpa who likes to drink water and he gets out of the tap," said resident Wally Davenport. "So we just told him and dumped out the water he was drinking so we'll have to go run back down and get some more bottled water."

The elderly, infants and pregnant women run the greatest risk of getting sick.

So, where does the bacteria come from?

"It has to come in through an opening in the system," Lillejord said. "Basically, the only opportunity of that is a storage tank where a vent has gotten into or some vandalism of that type and that's already been checked out we don't have that issue here."

It could be a false positive, meaning the original sample was contaminated somewhere in the process or just the faucet where the sample was taken is contaminated -- not the entire system.

E.coli are bacteria that typically live in the intestines of both humans and animals. It can cause diarrhea and abdominal cramps. And blood is often seen in the stool.

It can take 3 days or more for these symptoms to show.