Ten-year-old Sara Manhoff drank a glass of Hillfarm brand milk on April 2; three days later she became sick and still hasn't recovered.

"I'm not even sure what the expiration date was," says her mother Sheila Manhoff who bought the gallon of milk from a local grocery store, "because it was right before the Passover season and very hectic."

By the evening of April 5, "Sara woke up with stomach pains, started vomiting and had a temperature of 103," says her mother.

By April 9, one week after drinking contaminated milk produced by Jewel Food's Hillfarm Dairy in Melrose Park, Ill., both Sara and her sister Reena, 8, were in Lutheran Memorial Hospital suffering from dehydration.

They were victims of the largest salmonella epidemic in U.S. history. And nobody knows why it happened.

"The Chicago outbreak is unusual because of the numbers involved and because the pasteurization process has a great safety record," but salmonella outbreaks are actually quite common, says Dr. Scott Holmberg of the Centers for Disease Control in Atlanta. "There are between 2 and 4 million recorded cases of salmonella poisoning every year in this country."

Since March 21, more than 8,000 people in six states (Illinois, Wisconsin, Minnesota, Michigan, Iowa and Indiana) have been sickened by two separate incidents of contamination at the same dairy. Five people have died.

The initial outbreak was traced to gallons of Bluebrook 2-percent milk, the other brand produced by the Hillfarm dairy, bearing a March 29 expiration date. Jewel removed that product from its dairy and the shelves of its food stores. Officials, failing to locate the source of contamination within the plant itself, allowed Hillfarm to resume operations.

However, even as public health inspectors continued to search for the cause of the initial outbreak, the dairy continued to produce contaminated milk and Jewel continued to sell it to customers.

"Both March 30 and 31 milk samples of the Bluebrook brand were tested and were free of contamination," says Jewel spokesman Jack Modzelewski. "We thought this was an isolated case."

Unfortunately, it wasn't. Health officials began to receive complaints from people suffering with symptoms of salmonella infections after drinking Jewel's Hillfarm milk with various expiration dates.

By April 10, as more and more cases poured into health departments, Jewel voluntarily closed its Hillfarm dairy and removed all Hillfarm and Bluebrook products from the shelves. "Jewel has no plans to reopen the dairy until the source of contamination is discovered," says Modzelewski, "when and if that ever happens."

What people calling the Illinois Public Health Department really want to know, says spokeswoman Susan Wydell, is why it happened, and whether it will happen again.

"It's happening all around the country all the time," says the CDC's Holmberg. "It's always been a problem, but suddenly people are taking a look at it."

Last year there were 185 reported cases of salmonella in the District, 1,346 in Maryland and 1,255 in Virginia, according to area health officials who stress that many cases go unrecognized, unconfirmed and unreported.

Salmonella is one of the most common types of food-caused illnesses, according to a report issued in July 1982 by the Food and Drug Administration, and is most often found in raw meats, poultry, eggs, fish, raw milk and all products made from those foods.

The illnesses caused by salmonella poisoning usually occur 12 to 48 hours after eating, and the initial symptoms -- which are often confused with stomach flu -- include nausea, fever, headache, abdominal cramps, diarrhea or vomiting. Symptoms can last from a few hours to a few days, and mild cases are treated with bed rest and plenty of fluids. Some patients may ultimately suffer from dehydration and require hospitalization. Salmonella seldom kills, says Dr. Doug Archer, deputy director of the FDA's division of microbiology. Nonetheless, it is the most common cause of death of all food-borne illnesses because of the large numbers of people who contract it.

The very young and old and those already suffering from other ailments are the most susceptible to complications, which can include bacteremia (a bacterial infection of the blood that can cause collapse of the circulatory system), persistent colitis or reactive arthritis (a temporary form of arthritis that can last up to six months).

Archer also says that salmonella is not an uncommon cause of death in patients with AIDS, or acquired immune deficiency syndrome.

Although a patient might feel fully recovered from a bout with salmonella, the contagious bacteria can linger in the body for as long as five weeks. That means individuals who have recovered from the acute symptoms of the disease can still infect others.

The best way to prevent infection is to practice proper hygiene, which includes hand washing after using the bathroom, changing a baby's diaper or handling uncooked food. Because salmonella organisms multiply rapidly at room temperature, the FDA further recommends that foods subject to contamination be stored at 40 degrees or below and prepared at a temperature of 140 degrees or higher.

Most salmonella infections come from food, and more than 50 percent of the cases are transmitted through meat, pork and poultry -- not milk. "When an incident of salmonella is milk-related," says Holmberg, "the culprit is almost always raw or unpasteurized milk."

Holmberg says the problem in the Hillfarm dairy was probably the result of either inadequate pasteurization or contamination after pasteurization.

"We know pasteurization kills salmonella," he says. "It is an organism, just like a human being . . . Pasteurization provides enough heat to kill salmonella bacteria."

Health officials interviewed in the metropolitan area agree with Holmberg and are urging people to drink pasteurized milk rather than raw milk.

"Pasteurization has a good track record," says Dr. Carl Armstrong of the Virginia State Health Department. "Any changes in the process are unlikely." Lankford Hicks, an epidemiologist for the District, stresses that while there is always a possibility of a serious outbreak here or anywhere else, incidents of salmonella poisoning are not as widespread as they could be, and officials do a good job of controlling them when they do occur.

Meanwhile, Sheila Manhoff is serving milk to her family again. "I'm gradually reintroducing foods to my children," she says. "And Monday we tried milk -- with hesitation, I might add, but we tried." She says she uses less milk in preparing foods these days, but thinks her hesitation will pass with time. "Like my husband said, we can't live in fear all the time. We have to trust . . . and take things step by step."