More than a third of the students at a private school in the NYC suburbs are missing class because of an apparent virus.

Westchester Health Commissioner Joshua Lipsman says 325 students and 20 staff members were reported absent Thursday at the Hackley School, up by nearly 100 over Wednesday. The 109-year-old Tarrytown school has an enrollment of 836 from kindergarten through high school.

Lipsman says tests are not complete but the illness appears to be a highly contagious but fairly common virus. He says none of the victims are seriously ill.

The best measures to contain the outbreak are frequent hand-washing and decontamination of surfaces, and the school is encouraging both. Lipsman says victims should drink plenty of non-caffeinated, nonalcoholic fluids.