Deborah Copaken Kogan’s and 4-year-old son Leo, whose face is swollen, a symptom of Kawasaki Disease.

Facebook has touched and disrupted our lives in ways we can’t always put our finger on. But when a life is saved with the help of the social networking Web site, its influence is clear.

In 2008, a book set out to chronicles some of of the ways Facebook has transformed people’s lives: A woman found the kidney donor she needed to survive, long-lost sisters were reunited by chance, and a teacher in Denmark successfully invited the nation’s prime minister to visit his special-needs social studies class.

Now, the social networking site has helped saved a 4-year-old boy’s life after his mother, Deborah Copaken Kogan, got a diagnosis for his disease on Facebook.

Kogan told the full story in Slate Wednesday, which began on Mother’s Day morning, when her son Leo woke up with a rash she was sure was strep. While Kogan was at the doctor’s, she took a photo of Leo and posted it to her Facebook page, writing, “Nothing says Happy Mother’s Day quite like a Sunday morning at the pediatrician’s.”

Leo did not have strep, and his symptoms only got worse. Kogan posted more photos of Leo on her page to ask questions about her son’s swelling and fever.

The day after Mother’s Day, Kogan got a call from a friend who said her son had had the exact same symptoms, and was hospitalized for Kawasaki disease. Other Facebook friends wrote Kogan saying they thought it was Kawasaki, too.

It’s a disease attacked the coronary arteries surrounding the heart, and it is sometimes fatal.

Kogan rushed her son to the hospital, where it was confirmed that Leo did, in fact, have Kawasaki disease and needed to be treated for several weeks.

After Leo’s diagnosis, the support from Facebook friends continued, and Kogan wrote that Facebook made her “feel connected—profoundly connected—to the human race while living, breathing, eating and sleeping in the isolating, fluorescent-lit bubble of a children’s hospital ward, where any potential humans I might have ‘friended’ on our floor were too distraught over the fates of their own children to make any room in their hearts for strangers.”

After three weeks, Leo was released with a recovering liver and mostly healed heart. But for every year of the rest of his life, Leo will have to get echocardiograms, and Kogan acknowledges that at any moment, “my son could suddenly drop dead of a heart attack.”

“Such knowledge, to say the least, is isolating,” she writes. “But thanks to my Facebook friends and their continuing support, I do not feel so alone.”