Assumption officials issued a boil water advisory Friday afternoon for the entire parish after testing Thursday at a home in the Labadieville area turned up E. coli.

The precautionary order, which means residents must boil any water they plan to ingest, affects Assumption Parish Waterworks’ nearly 10,000 customers, or more than 20,000 people, parish officials said.

B.J. Francis, Waterworks’ general manager, said a follow-up round of tests was taken Friday, and the soonest the order could be lifted is likely Monday.

Escherichia coli is a bacteria commonly found in the intestines of people and animals. While some strains are harmless, others can cause diarrhea, respiratory illness and other ailments, the U.S. Centers for Disease Control and Prevention said.

Francis said a routine test on Tuesday or Wednesday turned up high bacteria content in the water, though not E. coli. A second required round of tests was taken Thursday in which officials retested the original site and sites upstream and downstream of that site on the water system.

While the original test site and downstream site were negative, the upstream site, an unoccupied home that still is an active water customer, had E. coli, Francis said.

Francis said he learned about the results Friday and issued the order. Officials are investigating to see why water from the home may have had E. coli, he said.

The Assumption system draws its water from Bayou Lafourche. Francis said that given the apparent isolation of the E. coli discovery, he doesn’t believe, at this time, the E. coli stems from the system’s water source.

Francis said he had considered a more limited boil water advisory but said state Department of Health and Hospitals officials wanted the parishwide order as a precautionary measure.

While a boil water advisory is in effect, DHH officials said, residents should bring water to rolling boil for one minute and then allow the water to cool for up to 30 minutes.

Water used for hand-washing dishes, brushing teeth, making ice, cooking or cleaning food that won’t be cooked should be boiled first or come from a bottle.

The Assumption water system’s 2014 consumer confidence report noted three times when monthly tests found chlorine levels — a key disinfectant for bacteria — had fallen below mandated levels and noted once when coliform bacteria had exceeded a key threshold.

Coliform are used as an indicator for other, possibly harmful bacteria, but testing that year never turned up E. coli, the report says.