[UPDATE: The photo of the bread experiment was originally taken by Courtney Lee Simpson, a pre-K teacher from Oak Ridge, Tenn. It has been shared and reposted hundreds of thousands of times, including by teachers like Allen, who then brought the experiment into their own classrooms.]

CHARLOTTE, N.C. -- A North Carolina teacher’s experiment about the importance of hand washing has gone viral.

After Courtney Lee Simpson, a teacher from Oak Ridge, Tenn., posted a photo of the bread experiment, Donna Allen, a health occupation teacher in Fayetteville, shared Simpson's photo on Facebook.

“I did this while teaching about germs and how they spread,” Allen explained in the post. “You use three pieces of bread. You let all the kids see you put a piece of bread in a baggy with a glove on hence 'controlled'. Then you wash your hands and put a piece of bread in a baggy for 'clean'. Last, but definitely not least, you pass a piece of bread around and let every kid in class touch it then you put it in a baggy and label it dirty. Watch how the bread changes over time due to germs.”

Indeed, the bread that had been touched by the students’ unwashed hands became extremely moldy in just a few days.

“I’m so technologically challenged, I had no idea. My daughter had to tell me it went viral,” Allen laughed. “It is surprising. I mean, it’s so simple.”

Allen told NBC Charlotte the experiment was a hit with her class.

“Instead of me just standing there saying ‘Wash your hands. Wash your hands,’ it gives them a visual,” she explained.

And the experts say it’s totally legitimate.

“I wish every teacher would do this with their students,” said Dr. Arash Poursina is an infectious disease specialist. “[The experiment is] very accurate, very legitimate, and I think very timely.”

With flu season upon us, he says the slightly icky, viral-in-every-sense-of-the-word lesson is more important now than ever.

“Spreading the word and education alone can reduce it so much,” he said. “It should just become a part of the school curriculum as far as I'm concerned. It was a teaching moment for the kids, but I think the take-home message is for everybody.”

Dr. Poursina recommends that the average person wash or sanitize their hands 10-20 times a day.