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Waterborne Diseases in California

Written by Ariyan Darvish, Edited by Rushlenne Pascual

Introduction 

Waterborne diseases are illnesses caused by the contamination of water by pathogens like bacteria, viruses, and parasites. They can be contracted by drinking or bathing with contaminated water and can also cause diarrhea and vomiting. Such diseases are usually more prevalent in less developed countries with lower quality water treatment and sewage facilities, however waterborne diseases are still present in California and it is important to understand them to prevent outbreaks.1 This article will discuss causes, symptoms, and treatment of several water borne diseases prevalent in California.


Legionnaires' Disease

Legionnaires' Disease is a waterborne disease caused by the Legionella bacteria. These bacteria live in natural freshwater environments as well as man made systems. It is a type of pneumonia and spreads through water droplets or mist from hot tubs, fountains, and water cooling towers. It does not spread from person to person, leading to only about 500 reported cases per year in California.2 However, outbreaks are commonly caused by a bacterial population contaminating a water source used by many, like when a contaminated cooling tower in Napa caused an outbreak in the county.3 Because of this, public health officials should focus on identifying and sanitizing contaminated water facilities when dealing with Legionnaires outbreaks. Symptoms of Legionnaires' Disease include: fever, shortness of breath, coughing, and headaches. These are very generic symptoms meaning diagnosis of the disease must come from laboratory testing like urinary antigen test to detect the Legionella antigens in urine. The disease is typically treated with fluoroquinolones or macrolides over the course of 7-14 days.4


Norovirus

Norovirus is a much more common water borne illness, infecting over 20 million people in the US each year. Norovirus is extremely contagious and spreads through vomit and diarrhea. In addition to being a waterborne illness, it is also considered a food borne illness. Symptoms include vomiting, nausea, diarrhea and stomach pain. According to the California Department of Public Health (CDPH), norovirus outbreaks in California are commonly caused by eating undercooked oysters that live in contaminated water. The CPDH has a shellfish and seafood advisories program where they monitor diseases like norovirus caused by seafood and issue warnings to prevent outbreaks. For example in May of 2025, the CDPH warned against the sale and consumption of oysters imported from Korea as many were contaminated with norovirus. This serves as an example of intersection between public health monitoring and environmental science. As no specific cure for norovirus exists, treatment consists of oral rehydration therapy or electrolytes along with drinking plenty of fluid to prevent dehydration as the virus causes rapid loss of water due to diarrhea and vomiting.5


Giardiasis

This waterborne illness is caused by the Giardia duodenalis parasite. The parasite can be ingested through contaminated water and colonizes the small intestine causing diarrhea, abdominal pain, bloating, and dehydration. Giardiasis is estimated to infect 1.1 million people per year in the US. Giardiasis has the most infections in the summer, likely because people engage in swimming and other outdoor activities that increase their chances of being exposed to contaminated water. High risk areas for infection in California include mountain streams and lakes in the Sierra Nevada, where the parasite can survive for months in cold water. To test for a Giardia infection, stool samples can be analyzed for presence of the parasite’s antigens or cysts. This can be done with Direct Fluorescent Antibody tests where fluorescent antibodies bind to specific Giardia antigens which would indicate the presence of Giardia. Giardia can be treated with antiparasitic such as Tinidazole or Metronidazole and takes a few days to a few weeks.6

References

  1. Foodborne and Waterborne Illnesses | Infectious Diseases. health.ucdavis.edu. https://health.ucdavis.edu/conditions/infectious-diseases/foodborne-and-waterborne-illnesses

  2. Legionellosis Information for Local Health Departments & Public Health Labs. Ca.gov. Published 2026. Accessed April 16, 2026. https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/LegionellosisLHDs.aspx 

  3. Legionnaires’ Disease | Napa County, CA. Napacounty.gov. Published 2023. Accessed April 16, 2026. https://www.napacounty.gov/3370/Legionnaires-Disease

  4. Mayo Clinic. Legionnaires’ disease - Symptoms and causes. Mayo Clinic. Published 2021. https://www.mayoclinic.org/diseases-conditions/legionnaires-disease/symptoms-causes/syc-20351747 

  5. California Department of Public Health. Norovirus. www.cdph.ca.gov. Published November 21, 2024. https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/Norovirus.aspx

  6. CDC. About Giardia. Giardia. Published May 3, 2024. https://www.cdc.gov/giardia/about/index.html

This post is not a substitute for professional advice. If you believe that you may be experiencing a medical emergency, please contact your primary care physician, or go to the nearest Emergency Room. Results from ongoing research are constantly evolving. This post contains information that was last updated in June 2026.


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