Residents in Bloomfield Township, New Jersey must boil their water because E. coli bacteria have been found in the system.
The bacteria are a sign the water may be contaminated with human or animal waste and can sicken people, especially those with weakened immune systems.
It's not known how the contamination happed.
The township, which is in Essex County, says contamination can occur when heavy rain causes increased runoff, a break in the pipes or a failure in the water treatment process.
The township's water comes from the Newark Water Department.
Officials are flushing and chlorinating the system and looking for the source.
Bloomfield residents should boil their water for a minute and let it cool before drinking, making ice, washing dishes or brushing their teeth.