A swim coach for James Oliver, 14, said that the teen collapsed at West Chicago High School Dec. 8.
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In a letter to parents, the principal at Geneva High School said Oliver suffered an AVM stroke and brain hemorrhage and was in critical condition until he died Sunday night.
A neurosurgeon at Northwestern Medicine explained that this very rare condition is usually congenital.
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"It's thought that most AVMs are present from birth, and so we see a lot of AVMs either being diagnosed incidentally or being diagnosed because they bleed. We see a lot of that in children, and teenagers and young adults," Dr. Matthew Potts said.
Oliver was a freshman at Geneva High School, and no one expected him to be gone so soon.
"He just brought me so much joy, as he did many other people," swim coach Jennifer Heyer-Olson said. "And I think when somebody passes away, you just wish more that you could have told them that."
The Illinois high school boys swimming season began Nov. 20.
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A family friend said the teen was born with a brain arteriovenous malformation, or AVM, a condition where medical experts say blood vessels in the brain don't form correctly.
"About half people diagnosed with AVM experience symptoms, while the other half don't experience any symptoms until a rupture occurs," Potts said.
On a fundraising page set up for the family, relatives expressed their deepest thanks to everyone who offered thoughts and prayers for Oliver.
The west suburban community learned of the loss when the high school's principal sent an email to students on Monday about the tragedy.
The email reads in part, "As a school community, our thoughts are with his brother, Sean, a GHS junior, his family and friends, and our hearts are mourning this loss."
For years, Heyer-Olson coached both James and his older brother, Sean, who is also a swimmer.
Both are members of the West Chicago Co-Op.
They trained in the pool at her home.
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She got to know them and their family well, so well that when her husband battled a life-threatening illness, the boys would come by to help out with yard work and chores.
"Just delightful to be around, happy, polite, doing good, just doing kid things," Heyer-Olson said.
Those who knew Oliver said he and his older brother were inseparable.
They say he was really smart, had a great personality and loved collecting and trading collectible cards.
He was also active in his church's youth group.
"I just throw myself into work, and hope that I can get distracted, but then when you come back to reality, it really hits," Heyer-Olson said.