The musician shared a picture on social media of his four-year-old boy in A&E with the rare illness.
Secondary drowning is a condition that predominantly affects children. It often occurs after a child has been swimming and inhales water while in a pool or the sea.
With the condition, water gets into the lungs and ultimately makes it impossible to breathe.
Simpson wrote about his son's experience with the condition on social media: "I wanted to share a terrifying experience we had recently with my youngest son Jago, in the hope that if it helps just one person avoid a similar situation, it will be worth it."
He then detailed what happened: "We were on holiday enjoying a morning in the pool. Jago is a strong swimmer for his age and he was swimming under water but suddenly came up spluttering and coughing.
"At no point was he left unattended, It was momentary, nothing seemingly unusual for kids to experience. We got him out and he seemed fine and happy and was completely normal for the rest of the day."
Simpson said symptoms came on hours later and his son was initially misdiagnosed with food poisoning before the musician told the doctor about the spluttering incident which caused him to tell them to go to A&E straight away.
Simpson said: "During the hour long journey to hospital, Jago’s condition deteriorated and he seemed to be drifting in and out of consciousness. He was rushed through to have CT scans and X-Rays and to our absolute horror, we were told that he had water in his lungs and was minutes away from pneumonia."
He also thanked the medical team that worked with his son to ensure he got better: "Jago was diagnosed with secondary drowning. He spent the next three days in hospital and I'm so thankful for the amazing care he received. It was the worst thing we have ever experienced. The scariest thing of all, is that had we not taken him to hospital when we did, the outcome could have been very different."
The Busted singer, who rejoined the band in 2015, also has another son with his wife Anna.
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