This article originally appeared on RealSimple.com. 

Can a camera flash detect an otherwise invisible cancer? A news story gone viral this week shows that it can—and that it even has the potential to save lives.

MetroUK reported last week that a quick-thinking dad in England recently noticed that, in photos of his 14-month-old son Jaxson, one of Jaxson’s eyes glowed white—rather than the typical “red eye” that most people have in pictures.

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After doing some research online, Jaxson’s dad mentioned the anomaly to his doctor. Soon, Jaxson was diagnosed with retinoblastoma, a type of eye cancer, and began undergoing chemotherapy.

Thanks to early detection, doctors say the tumor has already shrunk to a third of its size and they hope it will remain benign, giving Jaxson the chance at a long, healthy life.

Mohammed Jaafar, MD, chief of ophthalmology at Children’s National Health System, says this father’s astute observations likely saved his child’s life. “This is a common symptom, but many parents don’t give it a second thought,” says Dr. Jaafar, who was not involved in Jaxson’s case. “They wait to see the pediatrician and don’t mention it for several months, but we know that cancer really needs to be diagnosed and treated early.”

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Retinoblastoma is rare, and is usually diagnosed in children before age 5. It accounts for about 4 percent of all cancers in children under age 15, according to the National Institutes of Health, and is diagnosed in 250 to 350 children a year in the United States.

In healthy eyes, the red-eye camera effect occurs because light is reflecting off of pink-colored tissue in the retina, at the back of the eye, says Dr. Jaafar. The telltale white reflection that Jaxson’s father noticed—technically called leukocoria—occurs because the tumor grows in front of that pink tissue, and gives off a white reflection, instead.

“This is often the first symptom in children with retinoblastoma,” says Dr. Jaafar. “You might notice it when light hits the eye in certain ways, but it’s usually most obvious in photographs that used a camera flash.”

A white reflection in the eyes doesn’t always indicate an eye tumor, he adds. It can also be caused by a cataract, a detached retina, or inflammation of the eye for another reason. But regardless, the symptom is worth being checked out ASAP by a doctor.

Other signs of retinoblastoma can include crossed eyes or loss of vision in one eye, which parents may notice when playing games like peek-a-boo with young children. In very advanced cases, the eye can become enlarged and bulging.

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In many cases, retinoblastoma can be successfully treated if caught early. “In 2017, we have multiple treatments that can often times save the eye and definitely save the lives of these children,” says Dr. Jaafar. Most commonly, doctors use chemotherapy to shrink the tumor or cryotherapy to freeze and kill the cancer cells.

Jaxson’s dad isn’t the first parent to call attention to the importance of a seemingly meaningless camera glitch. In 2014, a 3-year-old was diagnosed with Coat’s disease after a family friend noticed her eye was glowing in a Facebook photo. Also that year, a father introduced a smartphone app—called White Eye Detector—to help other parents detect leukocoria after his own son was diagnosed with retinoblastoma.

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