The warning advisory was put in place on Monday due to a bloom of blue-green algae.
Officials are warning residents to avoid contact with water at Silverwood Lake. Blue-green algae poses a variety of health risks including rashes, mouth ulcers, vomiting, diarrhea and cold and flu-like symptoms.
Pets are especially susceptible because they drink the lake water and lick their fur after being in the lake.
Do not cook, drink or wash dishes with surface water from the lake. Common water purification techniques will not remove toxins from the water. Avoid swimming, wading and jet-skiing. Boating is still allowed.
If you, a pet or someone else comes in contact with the algae-contaminated water, seek immediate medical attention. Also make sure to contact the San Bernardino County Public Health Department.