A boil water advisory has been issued for parts of San Diego, California, and surrounding areas that will last through the weekend.

Originally issued in the Silver Strand area of Coronado and Imperial Beach, the advisory was expanded late Thursday to include parts of San Diego and Chula Vista.

California American Water said the advisory is in place "due to positive E. Coli results found in the drinking water."

"Affected customers will be contacted directly by Cal American Water Company with additional information," San Diego County Emergency said.

Residents are being urged to boil all water for three minutes, then let it cool before using, California American Water said.

"The California Division of Drinking Water in conjunction with California American Water, is advising impacted California American Water customers that reside in the above cities to use boiled tap water or bottled water for drinking and cooking purposes as a safety precaution," California American Water said in a release.

It's unclear if this stems from the effects of Tropical Storm Hilary, which hit the region on Sunday.

Most strains of the bacteria Escherichia coli (E. coli) are harmless and live in the intestines of healthy humans and animals.

But some kinds cause disease by producing Shiga toxin. The bacteria that make these toxins are called "Shiga toxin-producing" E. coli (STEC).

The symptoms of STEC infections can include stomach cramps, diarrhea and vomiting. Some infections are mild, but others can be life-threatening.

The CDC estimates that 265,000 STEC infections occur each year in the United States.