A grieving central California couple is mourning the loss of their 9-year-old son and warning other parents about the shocking cause of his death: a common cold virus.

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Milpitas resident Belle Ang warned if a cold virus could kill her healthy soon-to-be fourth grader, Tristan Ang, "It could happen to any kid," she told KTVU-TV.

"It's very unexpected. Very healthy and all of a sudden, he's gone," Belle Ang told the news station.

Belle Ang said her son died very quickly. He complained of a fever, headaches and nausea on and off the week before he was expected to take part in a taekwondo championship.

"We are just thinking it may be too tired, little bit dehydrated," the boy’s father, Mark Ang, said. "We've been giving him Tylenol and Motrin. It would go away but not totally, and it would come back."

Test ruled out a number of illnesses. Then, the Angs told KTVU, then on June 28, Tristan Ang was rushed to a hospital, where he died several hours later from brain swelling caused by the adenovirus.

Adenoviruses are common viruses that cause a range of illness, including the common cold, sore throat, diarrhea and pink eye, according to the Centers for Disease Control and Prevention.

“He didn't have signs of cold, nothing,” Mark Ang said.

Despite their grief, the Angs wanted other parents to be aware that what may seem like a normal childhood cold or sickness could quickly turn into a deadly situation.

The family set up a GoFundMe page to help pay for expenses.