One baby tragically died and eight others were admitted to hospital after being struck down with a virus that is typically harmless.
Last night, the World Health Organisation said there has been a rise in severe myocarditis - inflammation of the heart muscle - in newborns and infants.
Health officials said 15 babies, including 10 in Wales and five in England, had the condition between June 2022 and March 2023.
Eight babies were treated in intensive care, where they were intubated, put on a ventilator and received circulatory support.
One of the babies is still in hospital, according to an alert issued by the WHO yesterday.
Of the babies who had the condition, nine tested positive for an enterovirus, a common type of virus that usually causes mild illness.
Enterovirus is a common childhood infection that can cause a range of symptoms. It rarely affects the heart, however, in very young babies, enterovirus can cause severe illness in the first few weeks of life.
In South Wales, 10 babies aged under one month received hospital treatment for enterovirus and myocarditis and one of them tragically died, Public Health Wales said.
The sudden spike in the condition left health chiefs baffled and an investigation has been launched to determine the causes of the spread.
Previous cases of virus outbreaks among children, such as Strep A last winter, were blamed on lockdown restrictions weakening immunity.
There have been no reports of severe myocarditis since March - but cases appear to have peaked last November.
The WHO labelled the spike as "unusual" and said that if more cases emerge, childcare facilities and schools should close.
Parents Joann and Christian Edwards lost baby Elijah when he was just 12 days old, with doctors telling them he died of an infection so rare he was the only one in the country to die from it in years.
But just over a year after his death, they found out little Elijah is not the only Welsh baby to have died with the infection, and that multiple other babies had been fighting the rare condition.
Elijah was born healthy, and was discharged from hospital with Joann a day after he was born.
Coronavirus restrictions were still in place, which meant no other visitors saw Elijah in that timeframe other than dad Christian, as well as the health care professionals at the hospital.
Within a few days of being at home Elijah appeared to be lethargic and constipated, but the symptoms were put down to jaundice.
When he was a week old, he completely stopped feeding so his parents rushed him to A&E. He was first given a diagnosis of sepsis and then bronchiolitis. He was transferred to the University Hospital of Wales, where they picked up stress on his heart.
Joann said they were asked if there was a family history of heart conditions, and there was not. He was eventually moved again to Bristol Children's Hospital where they picked up the enterovirus, and were told it was a condition that is known to attach to the heart in newborns.
She said: "The consultant said it was very likely he contracted it within 24 hours of being born. I had never heard of it before. But we still don't have any answers. It has been hard.
"With my daughter, if we didn't have her, I don't know how we would have survived it, to be honest. When you have another child, you have got to keep going for them."
Joann said she was upset and angry to read about the fact that other cases of myocarditis and enterovirus were being investigated in Wales. "All this time we've been kind of ignored and told you're on your own, it is something that has just happened to you, and all of a sudden this has come out and actually this has all happened on our doorstep.
"I would have appreciated it a lot more if they had contacted us in advance. Just to let us know it was going to come out."
She said just a week or two before, Joann had chased Cwm Taf Morgannwg Health Board for the findings of an investigation they promised would happen into Elijah's death.
