The grieving family of a 9-year-old boy who died just a week after coming down with what they thought was a common cold is speaking out to warn others that “it could happen to any kid.”

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“He didn’t even have signs of a cold,” Mark Ang, Tristan’s father, who is a nurse, told KTVU.com.

The Angs, of Milpitas, Calif., said they gave their son Tylenol and Motrin and that his symptoms would lessen but then come back. Tristan’s mom, also a nurse, said prior to coming down with the illness her son had been “very healthy.”

“We’re blaming ourselves,” Belle Ang told NBCBayArea.com. “What did we miss?”

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The Angs told the news outlet that they took their son to the hospital a first time only to be sent home with a diagnosis of dehydration, but when they returned a second time, Tristan was admitted to the ICU. There, his father said his brain started swelling and his heart stopped four times.

“After the fourth go, that’s when the doctor told Belle and I that he’s dying,” Mark Ang told the news outlet. “As a nurse, I know. But as a dad, I don’t want to believe it.”

They told KTVU that tests at the hospital ruled out meningitis and that eventually, doctors landed on complications from the adenovirus.

"It could happen to any kid," Belle Ang said. "Even the healthiest kid could die or get the virus fast."

Adenovirus a common virus that causes a range of illnesses including sore throat, bronchitis, pneumonia, diarrhea and pink eye, according to the Centers for Disease Control and Prevention (CDC). They can cause mild to severe illnesses, although serious complications are considered rare. Those with weakened immune systems or existing respiratory or cardiac disease are considered at higher risk for developing serious complications.

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Tristan, who was a black belt in Taekwondo and a doting older brother, died on June 28.

"I guess God needs a warrior up there, it's so hard,” Mark Ang told KTVU.