There is a flu going around as is every year.
If there are too many students getting flu, the class will be closed in Japan.
If a student gets flu, he or she is forced to absent school for at least 5 days.
Many families with kids who go to kindergarten or elementary school experience spiral which a kid get flu from school and give it to other family member.
When one get well, other get bad and it continues until the end of the winter.