Cal Water has found traces of contaminants in the water of the South El Monte area of Los Altos.
The contaminant is confirmed as E. coli. Cal Water said the water from the South El Monte area has not spread to other areas nearby.
In a boil water advisory on Sunday, Cal Water officials recommend that residents in the South El Monte area boil their water for one minute before consumption. Customers should also dispose of any ice made with tap water or by their refrigerator.
If customers use bottled water, Cal Water will reimburse customers who provide a receipt, Kingman said.
Several residents on Monday said the warning should had come in a more organized manner.
Eve Farrington-Yen said the warning from water officials came too late for her 8-year-old son, Tyler.
"I was concerned because my son was sick the night prior to the warning," she said. "He threw up and I think it's because of the E. coli. He was very sick."
The water is still safe for showering, bathing and other uses in the home. Cal Water officials said customers should check with their veterinarian if they are unsure whether to give the home's water to their pet.
Cal Water said tests indicate that water in the affected area has E. coli and total coliform, bacteria that can make people sick. People with weakened immune systems are particularly at risk, as are infants, young children and the elderly.
Spokeswoman Yvonne Kingman said 853 customers have been affected and phone notifications have gone out to nearly 600. Cal Water workers are also notifying all customers by going door to door.
Cal Water will be notifying residents when the water becomes free of contaminants.
NBC Bay Area's Marianne Favro contributed to this report.