County and state health officials are investigating what prompted more than 100 students to call in sick over a two-day period at a San Diego-area high school.
More than 100 students called in sick Thursday and Friday at Poway High School.
A district spokesperson said the absences have been linked to some type of 24-hour stomach bug.
However, county health officials said they are not sure what caused so many students to develop similar symptoms.
What they do know is the absences were not caused by influenza, which is a respiratory virus.
County Public Health Official Wilma Wooten said the Poway High School case is a different type of virus that could be norovirus or gastrointestinal.
If there was no diarrhea reported by the affected students then it was less likely to be norovirus, Wooten said.
She added that officials won't know for sure until they get a specimen to make diagnosis.
Food poisoning was ruled out after talking with students and seeing if there was any connection to food and the illness, school officials told NBC 7.
County health officials were contacted as part of the protocol for that many absences in such a short period of time.
Wooten said her department and the Department of Public Health will work with Poway High School to determine who was sick, how it spread and what it was.
Administrators at the high school said attendance was back to normal Wednesday and that they are encouraging sick students to stay home from school.
"It's flu season. We make sure that we tell students if they aren't feeling well, not to come to school. Sometimes students feel the pressure to attend even if they aren't feeling well," said district spokesperson Jessica Wakefield.