The family of a three-year-old who died from Strep A have said he had "no symptoms" before suddenly collapsing while watching cartoons.
Theo Emm died at his home in Westbury, Wiltshire, on 18 February after having shown hardly any signs of illness, his family said.
He had been a "bit lethargic" the night before but felt better when he went to bed.
Parents Billy Emm, 29, and Jody Emm, 31, are now urging other parents to take action if they suspect their child is at risk.
The couple are still too distraught to speak, but Billy's brother, Chris Emm, has described the ordeal.
He said: "When he woke up on Saturday morning he wanted to watch the cartoons on TV so he was carried downstairs.
“Billy and Jody decided to phone 111 and while they were on the phone, Theo just collapsed with a cardiac arrest.”
A&E nurse Chris added: “They did say to me that they wanted more than anything to raise awareness and not for people to think it is a sob story to take pity on them.
"This kind of condition in children is often called a ‘silent killer’ and it is very rare for children to die from it.
A Wiltshire Air Ambulance emergency response team arrived at the family’s home in Meadow Lane within minutes of the call.
Paramedics carried out CPR on Theo for about half an hour but they were unable to revive him.
Theo died just a week after his brother Oliver was born and the family now treasure a photograph of the pair with Oliver in his big brother's arms.
Chris said: “They are still trying to process what has happened but having a new child is helping a lot."
He added: “Strep A has been around for years but unless you routinely swab a child you would never find it.”
Strep A is usually accompanied by flu-like symptoms consisting of a high temperature, swollen glands, an aching body or a sore throat known as 'strep throat'.
It can sometimes be accompanied by a rash that feels rough, like sandpaper (scarlet fever) scabs and sores (impetigo).
Following antibiotic treatment, symptoms typically resolve within one to three days but strep A can occasionally turn into a more serious and potentially deadly illness.