This is a detective story whose final chapter features six federal investigators dressed in bright yellow plastic suits. They looked like chickens, someone would say later. "Chickens investigating chickens."

One hot Monday morning last July, the investigators showed up at a Utah egg farm. Dressed in the yellow suits, their faces covered with masks, they looked ready for an important, and toxic, mission.This is a detective story whose first chapter began, as so many of these stories do these days, with diarrhea. In February 1997 in Hermosa Beach, Calif., seven people sick as dogs went to their doctors, and their doctors notified the Los Angeles County Health Department. Then, a few days later, the detective work began.

Sometimes people can't remember what they ate last week. Sometimes epidemiologists will ask them to get out their checkbooks and their day-timers, hoping to remind them of forgotten meals. Sometimes the epidemiologists will read them headlines from the newspaper from the days right before they took ill, hoping that this will remind them of a particular hamburger or some raspberries or a little salad of mixed greens.

Sometimes, when an outbreak is large, stool samples will be sent to special labs where "molecular fingerprinting" can determine if all the cases of a particular food-borne illness are from the exact same bug.

But the Hermosa Beach case didn't require high technology or even much digging. Six of the seven diners tested positive for salmonella enteriditis. Not too long before getting sick, all six patients ate at a restaurant famous for its breakfasts. All six, in fact, ordered the Eggs Benedict: a poached egg, the yellow a bit runny, covered with hollandaise sauce.

That was the easy part. It took five more months to trace back the eggs to their source - first to an egg distributor in Colorado, then to three egg brokers and finally to three egg farms in Utah. And that's when inspectors from the Food and Drug Administration showed up in their yellow plastic suits.

It's not particularly pleasant work because egg farms are populated by chickens. Chickens make manure, walk in manure, sometimes eat manure. The investigators dipped sterile gauze pads in condensed milk, then dragged the swabs along the places where bacteria might be hanging out: the floor, counters, the conveyor belts that carry away eggs and manure.

Two of the egg farms tested positive for tiny traces of salmonella enteriditis. There was no way of knowing, though, if the eggs that made the people in Hermosa Beach sick came from either of these two farms - because by the time the swabs were taken, five months had transpired. Any eggs that might be from the same batch were long gone.

But the Utah eggs had been implicated, nonetheless. The FDA ordered one farm to send all its eggs to a breaking plant to be pasteurized. It also ordered the other farm to kill all the chickens in the infected coop, about 20,000 in all.

Those seven Eggs Benedict and all that happened afterward are a microcosm of everything that is right, wrong and inconclusive about America's food supply.

Are the eggs we eat safe? The ground beef, the lettuce, the chicken, the sprouts? Are government and the food industry doing enough to protect us? Are the regulations and trace-backs and recalls and new money being poured into new food safety initiatives really doing us any good?

The answers are fuzzy.

And so there is a vague disquietude in the land. We go right along eating, of course. We hear about the big outbreaks - the Hudson beef, the Guatemalan raspberries, the apple juice that killed that little girl in Colorado - and we feel anxious, at least for a while. The names of bacteria - salmonella, E. coli - have begun to sound as familiar to us now as the names of our rock stars.

Is our food safe?

Yes and no.

On a disaster likelihood scale - where, say, earthbound asteroids are on one side of the spectrum and an El Nino-caused snowstorm is on the other - food-borne illnesses lie somewhere in the middle. Are you and your children likely to get deathly ill from your dinner tonight? No. Should you still take precautions? Definitely.

There are dozens of vantage points from which to explore the complexities of food safety. Eggs are one of them.

"We used to think an egg was perfect," said Gene Gregory.

"We thought, by the way it was made, that bacteria couldn't grow in it." Gregory is vice president of the Atlanta-based United Egg Producers, a group that represents the egg industry.

It used to be that if an egg eater became sick with salmonellosis, it was assumed the bacteria came from the shell, contaminated perhaps by chicken manure or maybe rat droppings.

What egg growers and scientists know now, though, is that salmonella enteriditis can be present inside the egg - having been transported there from the ovaries of the hens that laid them.

According to the Centers for Disease Control in Atlanta, cases of salmonella enteriditis infection are up since the 1970s. In those days, you could let your children eat raw cookie dough or order the Caesar salad without asking first if the restaurant uses pasteurized eggs.

In Utah, epidemiologists noticed a fairly significant rise in salmonella enteriditis in the fall of 1995. State epidemiologist Craig Nichols invited the CDC in to help investigate the increase.

In its most benign forms, salmonellosis is just an unfortunate, painful case of diarrhea. But it "is often not a trivial disease," notes Stanford University microbiologist Stanley Falkow. It "can be fatal to people at the age extremes" and to people who are already weakened by cancer, HIV or other diseases. Salmonella kills more people each year than E. coli O157:H7, although the latter has gotten more press lately.

Salmonella's deadly potential is the reason epidemiologists try to track down a situation such as Utah's salmonella enteriditis increase in late 1995. After scores of phone calls, the Utah and CDC investigators determined that the culprit was the habit of some Utah restaurants to "pool" eggs in a bucket and then to draw from that pool to make sauces, omelets, dressings, French toast. It's a bad practice because just one contaminated egg can infect the whole batch.

So in 1996, the rules were changed. Restaurants are now allowed to pool only three eggs at a time.

That was one way of fighting food-borne illness at one end of the food chain.

The Hermosa Beach incident a year later was another way. Instead of simply taking action against the restaurant - which also pooled eggs - the Food and Drug Administration stepped in to trace the eggs back to their source.

The federal trace-back still rankles staff members of the Utah Department of Agriculture and Food (which is as interested in protecting Utah's food industry as protecting Utah's eaters). The trace-back also miffs egg producers.

"I bet if I swabbed your office, I'd find salmonella entereditis," egg lobbyist Gregory tells a reporter.

His gripe is that an egg farm can have traces of salmonella - on floors, belts, fans - but that doesn't mean that any eggs are necessarily infected. One study of infected flocks found that only one in 10,000 eggs actually contained the bug. And, he said, the majority of egg farms are not infected. If you take all eggs - produced on infected and non-infected farms - the likelihood of getting a salmonella-laced egg is one in 235,000, he said.

Rodrigo Villar, an epidemiologist with the CDC, offers different numbers. One study, he said, found that 85 percent of America's egg farms have salmonella enteriditis.

As a result of the Hermosa Beach outbreak, Utah egg growers have adopted new guidelines to reduce the number of microbes on their farms. Under the agreement, each egg farm will come up with a quality assurance plan, which will be monitored by inspectors from the Utah Department of Agriculture and Food. The farms will do random, periodic testing for salmonella, will increase rodent reduction and agree to disinfect areas before new flocks are introduced. Most Utah egg farms were doing all this anyway, said Michael Marshall, Utah state veterinarian.

Does all this mean you'll never have to worry again about the eggs you eat? "Are eggs guaranteed to be without bacteria in every situation? No," said Marshall.

Can an egg farm ever guarantee, even if it brings in disease-free hens, that salmonella enteriditis is gone for good? "You're thinking like a city person," said Marshall.

"Do chickens eat and do they defecate?" he asks. Does the wind blow, do rats scurry through hen houses, do workers sometimes bring in germs on their boots?

And what about the chickens we eat? Consumer Reports magazine last month reported its survey of 1,000 supermarket chickens in 36 American cities: 63 percent of the chickens were infected with campylobacter jejuni and 16 percent with salmonella.

The magazine's recommendations: require testing for campylobacter at poultry plants (only salmonella is currently required); and lower the acceptable limits for salmonella infection (currently, a plant can have 20 percent test positive).

America's chicken farmers and processors could do more to keep us safe, argue people like Ned Groth (the Consumer's Union biologist who authored the report), and Caroline DeWaal of the Center for Science in the Public Interest.

In late March, the FDA approved the use of a product designed to reduce the number of salmonella microbes on chicken. The spray, called Pre-empt, works by covering newly hatched chickens with a solution of 29 "good" bacteria. When the chicks ingest the solution (by pecking at their wet feathers), the good bacteria grow, preventing the growth of salmonella.

If it works, will that be the end of salmonella in chickens? Or will new, trickier bacteria evolve to take its place?

"We have the safest food supply in the world. But the world is not sterile," notes Becky Shreeve, the Utah Department of Agriculture and Food's food program supervisor. It's a theme you'll hear again and again if you talk to the growers and the regulators of America's food supply.

"The meat industry is the most highly regulated industry on Earth, and the United States is the finest of the best," said Utah state veterinarian Marshall. "You have more risk (of getting sick) by eating your fingernails than eating a piece of chicken. But there are no guarantees in life."

Food regulators call chicken and eggs "potentially hazardous products." Cook them well, they'll tell you. Don't cross-contaminate. And take a cue, perhaps, from Penny Adcock, an epidemiologist with the Centers for Disease Control:

Faced with the prospect of eating French toast or Caesar salad or a tasty Bearnaise sauce at a restaurant, Adcock always asks the waiter if it was made with pasteurized eggs.

Next: The bacterial villains that make us sick and the people who fight them.