When nearly half of the 150 children enrolled in a summer day-care program in Florida became ill shortly after lunch, Orange County health officials thought a case of mass food poisoning, perhaps from spoiled ham-and-cheese sandwiches, had struck.

Not so, according to a Centers for Disease Control review of the July 26, 1989, incident. Instead, epidemiologists believe, the children unconsciously mimicked the illness of one student who complained that her food "tasted bad" and vomited.

"We did not find any other logical explanation," said Jean-Claude Desenclos, a CDC epidemiologist assigned to the Florida Health Department. "The group reacted as a group to a particular situation. It's something we don't understand completely."

The reaction, known as mass sociogenic illness, occurred within two to 40 minutes after lunch and involved 63 children, mostly girls between the ages of 4 and 14, according to the CDC study, published in its May 11 Morbidity and Mortality Weekly Report.

More than half of the affected children suffered cramps, nausea and headaches and two thirds experienced vomiting. A smaller percentage had additional symptoms, such as dizziness, malaise and sore throat. None of the children, evaluated at local hospital emergency rooms, had symptoms after two hours. More than 90 percent of them returned to the day care center the next day without incident.

The outbreak began when one 12-year-old girl became ill after eating a prepackaged lunch of a ham-and-cheese sandwich, diced pears, chocolate milk and apple juice. When other children developed similar reactions, some of the staff suggested to the children that the food may have been spoiled.

However, food samples tested by the Food and Drug Administration found no toxins, such as salmonella, or pesticides. Nor were unusual problems with the ventilation system at the day-care center discovered, CDC investigators said.

Instead, they theorize, the children may have reacted because they were told the food was contaminated or saw other children vomiting.