Portland’s water bureau is warning that residents who have weakened immune systems should talk to their doctors before drinking city tap water, after tests from the Bull Run Watershed found two units of cryptosporidium in a 50-liter water sample Tuesday.

Water bureau officials said drinking from the tap should be safe for most people. There’s a small risk of diarrhea, vomiting and other stomach problems, but people with healthy immune systems should recover without medical treatment.

People with AIDS, cancer or other immune conditions could be at risk of life-threatening gastro-intestinal illness if they ingest the water. The water bureau suggests they should consult with medical professionals about whether it is safe for them.

The warning extends beyond Portland city limits to other communities that obtain all or some of their drinking water from Bull Run, including Sandy, Tualatin, Gresham and surrounding water districts that collectively serve close to a million people.

Portland is under state and federal orders to start treating water, and has plans to build a treatment system by 2027 at an estimated cost of about $850 million. 

Correction: An earlier version of this story incorrectly identified cryptosporidium as a bacteria. OPB regrets the error.