Snow days are common at schools in Greater Boston. But at Chelsea High School, there’s a new reason for an unexpected break: Fly days!

An infestation of the pesky insects is to blame for Chelsea’s decision to close the school Thursday and Friday. Here’s a Q&A to bring you up to speed.

When did this happen?

It’s still a bit fuzzy, but the city is working hard to rid the school of the insects. In a letter to the school community Wednesday, Chelsea Superintendent Mary Bourque said the school noticed the flies Dec. 5, and a contractor was called in to clean the kitchen drains and grease traps in hopes of solving the problem.

It didn’t work, so the school called in an exterminator to search for other sources. But the inspection turned up more entry points for flies.

How did the flies get in?

Chelsea officials said Thursday they believe a break in pipes underneath the foundation of the school might be to blame. City Manager Thomas G. Ambrosino said exterminators and contractors are using tiny cameras to pinpoint the problem.

“It’s just a matter of televising all of the lines and finding the break,’’ Ambrosino said.

What kind of flies are they?

Chelsea has not said; there are thousands of species of flies. Metro Minute consulted Jeffrey Bain, an entomologist and owner of Bain Pest Control Service in Lowell, who has worked with monitoring and inspecting flies in schools (though not currently in Chelsea).

“If it’s a drain pipe, or if there’s a broken pipe under the floor, it could be drain flies,’’ Bain said.

His company’s website said the species feeds on organic matter and sewage, and can “breed in large numbers in the proper conditions.’’

He said school officials were correct that any solution would require finding the broken pipe.

“Organic matter gets stuck in those areas, and with the pipe decaying, it’s not going to resolve the issue,’’ he said.

Dylan McGuinness can be reached at dylan.mcguinness@globe.com. Follow him on Twitter at @DylMcGuinness.