The winter of 2017-18 is seeing one of the worst flu seasons on record, with 49 states seeing substantial flu activity and 39 of those seeing “especially high activity,” according to the U.S. Centers for Disease Control and Prevention. Schools, which are particularly prone to disease spread due to their crowded conditions and shared desks and materials, have even been closing in at least twelve states. There are just too many teachers and students sick.

For instance, at Saint Thomas Aquinas High School in Dallas Texas, more than 80 out of the 815 students were ill on Monday, January 29, so officials closed the school for two days and ran a deep-clean to hopefully reduce further spread.

A school district in Gulf County, Florida, reported more than 20 percent of students absent with flu-like symptoms.

Dr. Daniel Jernigan of the CDC says that the culprit this year’s flu season is the H3N2 strain. Similar to the “swine flu” from 2009, it has been hitting patients over 65 hardest, but plenty of younger patients have been affected. Due to a quirk of the disease, it’s also not very susceptible to any of this year’s flu vaccines, with a prevention rate between 10 and 30 percent. (The usual prevention rate is approximately 60 percent, according to the CDC)

Let me stress that this is no reason to not get your flu shot. There are multiple strains every year, and most are less virulent than H3N2. Vaccines are the second most important tactic in curtailing the spread of flu through a population, after isolation.

Schools like Saint Thomas Aquinas that closed got back to business fairly quickly, as soon as enough students and teachers recovered to fill the seats again. However, administrators hope that the brief break is enough to impress upon all how important it is to keep your flu home, and not try to muscle through it.