Medical science has long known about the effects of nitrate contamination -- dizziness, upset stomach, shortness of breath, lung infections, diabetes, possible links to cancer and potentially fatal blue-baby syndrome.
But county and state health departments don't routinely track these problems, as they do Valley Fever, flu, tuberculosis and human immunodeficiency virus. The nitrate problems are more subtle, and they're not specifically listed among reportable diseases for California.
So there are no official answers when someone asks how many babies have died or how many people have gotten cancer linked to nitrate-laced water in Tulare County.
People who live in small towns with the tainted water say they've seen plenty of evidence that children suffer.
"I talk to a lot of people who have children with stomach aches and dizziness," said Maria Herrera, an outreach worker for the Visalia-based Community Water Center. "I grew up in small towns around here. People talk about it all the time."
Doctors began diagnosing blue-baby syndrome in the 1940s. They recognized that nitrates interfere with the ability of red blood cells to carry oxygen. In babies, cells throughout their bodies are deprived of oxygen, giving their skin a bluish tint.
But the tint may not occur, or it may be too faint to see, especially in babies with darker skin. And often the other symptoms resemble more common stomach infections that cause diarrhea or vomiting. Medical experts say the disease probably often is misdiagnosed.
Nitrates are natural in humans -- they're in vegetables. The chemicals are essential to plant growth, but in excess, they can trigger stomach upsets.
Nitrates in well water come from rotting plants, fertilizers, sewage systems and runoff from animal wastes. They seep into underground water.
They're found in water all over the globe. In the Midwest, researchers estimated 40,000 infants were living in households getting well water that exceeded federal standards for nitrates.
In the San Joaquin Valley, nitrates are found in wells from Stockton to Bakersfield -- more than 20,000 square miles. One state study of private wells in Tulare County showed 70% had nitrate problems.