People in the United States have gotten used to the repulsive fact that raw chicken, meat and eggs are often contaminated with dangerous bacteria. Scrub the cutting board, we are warned, don't nibble the cookie dough, don't eat burgers rare. In other words, handle meat like a biohazard �" and then eat it.

But until recently, getting sick from salad was something that most Americans didn't even think about unless they were traveling to a poor country. At home, fruits and vegetables have been regarded as clean and safe for as long as most people can remember.

Lately, though, produce has caused a disturbing number of disease outbreaks; just since September, bacteria-tainted tomatoes, spinach and lettuce have made hundreds of people sick, and killed three. There have been 20 serious outbreaks in the past decade or so, and many have come from crops grown in California, not from imports. Fruit juices, alfalfa sprouts and almonds have also been involved �" all of them supposedly health foods, like salad, the things we feel most virtuous about eating.

The known outbreaks are just the tip of the iceberg, health officials say; far more illness is never reported. Most people don't call the health department about a few days of gut trouble. The government estimates that over all, food-borne microbes �" not just the ones on produce �" make 76 million people a year sick, put 325,000 in the hospital and kill 5,000.

In a modern country, a rise in disease caused by tainted food seems like a giant step backward in public health. But there hasn't been much public outrage or even disgust at the notion of filth seeping into the food supply.

Among the nastiest bacteria is E. coli 0157:H7, which makes a powerful toxin that can cause severe illness and sometimes even kidney failure. This is the germ found on spinach a few months ago, and more recently on iceberg lettuce served at Taco Bell restaurants. It comes from cow feces and was first identified in 1982. Feeding the animals grain instead of hay seems to promote its growth.

The strain is harmless to cows, but in people it is so dangerous, according to the Food and Drug Administration, that swallowing as few as 10 organisms may be enough to cause an infection. About 73,000 people a year get sick from this type of bacteria, and 61 die, the Centers for Disease Control and Prevention reports.

"It's gotten more attention this fall, but we've seen these outbreaks due to lettuce and other leafy greens for a long time," said Dr. Christopher Braden, chief of the outbreak response and surveillance team for enteric diseases at the disease centers. "We are seeing this on an ongoing basis. That's not an acceptable outcome. We need to find ways to interrupt that contamination."

Last August, the F.D.A. announced a "lettuce safety initiative" in response to recurring E. coli outbreaks. It began with last fall's lettuce harvest and included visits by inspectors to farms and cooling and packing facilities. But the spinach and Taco Bell outbreaks happened anyway.

There are several ways that bacteria can contaminate lettuce. Water is an obvious route, whether from unsanitary irrigation or spraying, or from flooding. Animals can carry bacteria onto farmland, which is apparently how the spinach outbreak occurred �" feral pigs wandered from cow pastures to spinach fields, taking E. coli with them. Sick workers who handle produce can also contaminate it, and so can dust blowing off pastures. One bad batch can spoil others when they are mixed for chopping and bagging.

Scientists think most contamination lies on the surface of crops, but studies have shown that it is possible for bacteria to be taken up through root systems and actually wind up inside the plants, where no amount of washing could get rid of it. In any case, E. coli 0157:H7 tends to be sticky and is difficult or impossible to wash off, even when it's only on the surface of produce.

Over the past 30 years, diseases linked to produce have increased, Dr. Braden said. Increased ability to detect outbreaks may explain part of the increase, but not all of it, he added.

"We're convinced it's real in large part," he said. "We're seeing an increased number of outbreaks, an increased number of cases in outbreaks, and an increase in the number of types of produce involved."

The reason is not known for sure. But, Dr. Braden said: "The way produce is farmed and processed has changed. It's become more centralized, and you have these huge processors and distributors that produce tens of thousands of pounds of a particular produce in a particular day. If something goes wrong with that produce you've got a big problem, whereas with small farmers, if there is a problem it's much more limited."

In addition, he said, bagged and prewashed produce didn't exist 25 years ago, and people today eat more raw vegetables than in the past.

"There's probably more susceptible people eating those things," Dr. Braden said. "We have an aging population, and more people with chronic medical conditions that might make them more susceptible."