What this teacher did for one of her students is extraordinary.

There are teachers who go above and beyond, and then there’s Cindy Santos.

Fate was at play on the day Cindy Santos was called in to a substitute teaching job at a Pennsylvania school. It was September, and she was looking forward to teaching kindergarten that day.

As she was being given her instructions for teaching the class, she was told about a little girl called Katelynn who was a little quiet and would need extra help. Katelynn Ernst suffered from hemolytic uremic syndrome, which is a chronic kidney disease. At the age of five she’d spend way too much time in hospital and was one very sick little girl.

"I was drawn to Katelynn," Cindy later recalled. "She was quieter than the others but had a sweet smile and engaging eyes."

At that stage Katelynn was having to undergo dialysis for 10 hours a day and was awaiting an urgent kidney donation.

Cindy says Katelynn was still on her mind days later when later when she came across a Facebook page called Katelynn's Kidney Journey. She immediately recognised the little girl from her class. The Facebook page had been been started by Katelynn's parents, searching for a kidney donor for their little girl.

Cindy, a married mother of three, read through the requirements and told her husband what she was thinking of doing.

She underwent initial testing and then further testing before the donation was approved. The chances of Cindy being a match for Katelynn was 1 in 80. The transplant took place in December.

These days, Katelynn is a normal, healthy little girl enjoying life. Her life was saved by a virtual stranger, a kind-hearted substitute teacher who was in a position to help and so she did.

Cindy not only received the reward of having helped save Katelynn's life, she also improved her own health in the process. It was during surgery that doctors discovered Cindy was suffering from a rare medical condition called 'pelvic congestive syndrome'. The removal of her kidney immediately stopped the stomach pains she'd been suffering from for most of her life.

"It's incredible to be able to do it," Cindy told Berks-Mont News. "It’s surreal that she has one of my organs and she’s thriving on it."

Katelynn's mother Alicia thanks God her daughter's path crossed with Cindy Santos. “We feel exactly the same way. God sent her to us," she told The Blaze.

Images courtesy of US Today and Katelynn's Kidney Journey Facebook page.

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Katelynn has recovered from her surgery. CLICK THROUGH these photos to see just how well she is doing: