A Marin County, Calif., elementary school followed all COVID-19 protocols when children returned to school in August. Masks for students and staff were required. The desks had been placed 6 feet apart. Social distancing was encouraged within the school. Basically, all the recommendations from health experts were being followed.

Little did the school know that it would become a COVID-19 case study for the Centers for Disease Control and Prevention.

One day, a teacher — who had not been vaccinated against the coronavirus — felt sick, according to Education Week. Fatigue, nasal congestion — you get it. She pushed it off, claiming it to be allergies or something of the sort.

She decided to read a story to her class. So she removed her mask.

Two days later, she tested positive for COVID-19. Those symptoms likely weren’t allergies, but the coronavirus.

And then half of her class had been infected. Almost all of the students sat two rows behind her desk, according to Education Week. The outbreak had begun to spread to other areas of the school, too.

Students, staff, siblings and parents became quickly infected — regardless of vaccination status, according to The Washington Post.

No one required hospitalization, per Education Week, which is good news.

Tracy Lam-Hine, an epidemiologist for the county, told The Washington Post that this isn’t the teacher’s fault. The teacher had only removed her mask for minutes, not hours.

“The mask was off only momentarily, not an entire day or hours. We want to make the point that this is not the teacher’s fault — everyone lets their guard down — but the thing is delta takes advantage of slippage from any kind of protective measures,” Lam-Hine told The Washington Post.

That’s why the Centers for Disease Control and Prevention has been adamant that the delta variant can spread rapidly in school environments, especially when staff and students are unvaccinated.

This case “highlights the importance of vaccinating school staff members who are in close indoor contact with children ineligible for vaccination,” according to the CDC.

The CDC recommends anyone who can get vaccinated to get their shots. It may save infections across so many people.

“If you’re vaccinated or unvaccinated, wear a mask in public indoor spaces in areas with high or substantial spread,” the CDC said.