Few members of the Harvard community can forget December 1994, when approximately 300 students--mostly first-years--fell mysteriously ill after dining in the Freshman Union.

Now, the initially elusive illness has been brought to light in the current edition (April-May 1996) of The Journal of Infectious Disease.

University Health Services (UHS) Director David S. Rosenthal '59 worked with colleagues at UHS, the Centers for Disease Control and Prevention (CDC) in Atlanta, the Massachusetts Department of Public Health and the Harvard University School of Public Health, to study the illness identified as viral gastroenteritis.

"Small round-structured viruses (SRSVs) were detected by electron microscopy in stool and vomitus specimens taken from ill students," said Rosenthal.

The prototype strain of the SRSVs is the food-born Norwalk virus. SRSVs rather than bacterias are the major causes of outbreaks of gastroenterits.

Blood samples drawn from students at the onset of their illness and then later during its convalescent phase revealed antibodies specifically against the Norwalk virus.

Once the single SRSV virus strain was identified, the researchers aimed to pinpoint its source.

"We used polymerase chain reactions (PCRs) to multiply the viral products and sequence the viral units," Rosenthal said.

"This novel molecular method allowed us to conclude that this virus was structurally the same virus as the one identified in the stool of a particular dining hall worker," he added. "This worker got sick after preparing the food item eaten statistically more by students who got sick than those who did not get sick."

The culprit food item--the vehicle for the virus transmission--appears to have been the salad.

"Despite the 187 menu items included in a survey given to 363 students, of whom 52 percent had gastrointestinal symptoms and 48 percent did not (controls), only eating items from the salad bars were significantly associated with illness," Rosenthal said.

The survey was distributed December 8 of 1994 to residents of student dormitories to determine illness characteristics and food consumption histories over the 3-day period from December 4 through 6.

It is interesting to note that salad bars as a whole--and not the individual salad bar items--were significantly associated with symptoms of gastroenteritis.

Rosenthal finds much enlightenment--both practical and purely scientific--from this study.

"We learned that there is a need to be more careful about workers coming in sick," he said.

However, being cautious is not an easy task since a person can be virally infected and thus contagious 24 hours before he or she exhibits acute symptoms.

Rosenthal also added that this study can now be "used as a model of how to do a nice epidemiological study of a gastroenteritis."

"Such investigation, in conjunction with molecular diagnostics, may enable early identification of sources of infection and improved outbreak control," he added