Edward Foxall was left scratching and covered in spots after contracting the illness from sibling and best pal Alfie, three, just days after his first birthday.
And while Alfie recovered quickly, his baby brother's condition deteriorated and he was rushed to hospital when he became breathless at home.
Chest x-rays revealed his right lung was filled with infected fluid, and he'd developed necrotising pneumonia - a rare complication of chicken pox, eating away at his lungs.
Parents Laura, 29, and Kieran, 28, an RAF chef, feared he wouldn't recover - and little Edward spent five days fighting for his life in intensive care.
But after five weeks of antibiotics he's finally home, and mum Laura is raising awareness of the rare complication the common ailment.
Mum-of-two Laura from Peterborough said: "Edward got very low very quickly when he was ill - and they say about babies that if they go that low that quick, they go high again very fast too.
"The speed at which the infection took hold was terrifying.
"When we were told that Edward would have to go to Addenbrookes for intensive care, I hit rock bottom. That was the lowest moment. I was really upset.
"I genuinely wasn't sure if he was going to come back.
"We might have been telling a different story if he hadn't been in hospital the night his lung filled with fluid and the infection really took hold."
Edward caught chicken pox from his older brother on May 20, and three days later his parents called 999 when he had trouble breathing and his temperature soared.
Doctors at Peterborough City Hospital did two chest x-rays which revealed he his right lung was filled with infected fluid.
Doctors told Laura and Kieran that Edward had "most likely developed a serious bacterial infection and sepsis from the rapid onset of the infection".
Poorly Edward was intubated and rushed to Addenbrookes PICU where they drained the fluid away.
The infection was confirmed as necrosis in the lung, a 'flesh-eating' disease which destroys body tissue and can enter the bloodstream.
To kill the infection and save his life, the baby boy was put on three types of IV antibiotics, one type of IV anti-viral medication, and had a feeding tube.
He spent a total of five days in intensive care, two-and-a-half weeks in hospital, and five weeks and two days of antibiotics.
Stay-at-home mum Laura said: "When he first got chicken pox, I didn't think anything much of it because Alfie's had cleared up quite quickly.
"I'd thought he'd pick them up from Alfie, but never imagined it would get bad.
"Initially, me and Kieran didn't think it wasn't anything too serious. It was pretty much like a normal bout of chicken pox, to be honest.
"On the second day, he was quite grumpy. He couldn't sleep, which was the first cause for concern, as Edward's a good little sleeper.
"After I gave him a bath on the third day, we had cuddles, and then I noticed that he was really tucking his tummy in to breath - which was because he was struggling to breath.
"I called 111 straight away, and they came quite quickly."The first chest x-ray didn't show any signs of something more serious, but the doctor at hand wasn't so sure so they advised us to stay overnight.
"We had a second x-ray the following morning, and that's when they told us that Edward had a serious bacterial infection, which turned out to be necrotising pneumonia.
"They also told us the pneumonia had then led to sepsis."
Though most people are unlikely to get an invasive infection, those with conditions like cancer or diabetes - or breaks in the skin or chicken pox - are at greater risk.
Now, Edward is making a fast recovery, and is back playing with his older brother Alfie.
"Alfie absolutely adores him, and can tell that Edward's better," she said.
"Edward had had a cold before, but he'd never been really ill with anything."Luckily, he has a strong constitution and is doing well.
"He's eating properly again, and is sleeping. He's quite an independent, happy baby.
"His weight is slowly getting back to normal and he's getting his strength back.
"He's been incredibly lucky, and we were incredibly lucky that staff all the way through