A 5-year-old boy died suddenly while on a family vacation after a stomachache turned out to be an extremely rare disease.

“5 days in the hospital and he was gone,” said Terese Peden, the grieving mother of Justin Vu, in an interview with FOX 8.

Denver-based Peden, her husband Viet Vu, and their children were in Oregon to visit family in late June when, on the fourth day of the trip, Justin complained of a stomachache, she said.

That quickly progressed into nausea and extreme lethargy and, on July 3, Peden took Justin to the hospital, where she said his condition worsened.

Doctors were stumped and initially thought it might be the boy’s appendix and transferred him to a Portland children’s hospital, before he was admitted into the ICU and put on life support.

Justin was eventually diagnosed with Systemic Capillary Leak Syndrome (SCLS), also known as Clarkson’s Disease.

This is an extremely rare and grave medical condition where plasma leaks from the capillaries into the body’s tissue, causing severe swelling and blood pressure to drop.

Fewer than 500 cases of Clarkson’s disease have been reported worldwide since the 1960s, according to the National Organization for Rare Disorders.