A 12-year-old girl has died from a rare skin and respiratory infection caused by bacteria common in many people.
The unnamed girl from Cilfrew near Neath died on Tuesday. She was called "delightful" and "hardworking" by the head of Llangatwg Comprehensive School.
The National Public Health Service for Wales said it was "rare" for invasive Group A Streptococcus to cause life-threatening diseases.
At present it does not believe there is a risk to other pupils.
The girl who died was in year 7, and had only been at the school for a term after moving from primary school.
Dr J�rg Hoffmann, consultant in communicable disease control, said: "We suspect that this child's death was caused by an infection with invasive Group A Streptococcus (iGAS), a bacteria that many people carry and which can, on rare occasions, cause life-threatening diseases.
No more than a handful of cases are normally reported in Wales in the winter months. There are no figures available for how many deaths are caused each year.
Dr Hoffman added: "It is very rare for healthy people to catch iGAS from someone else with the same condition, and we will treat close contacts with antibiotics only if they are displaying symptoms.
"At present our investigations show that there is no risk to other children attending the same school, or to the wider community.
"We are continuing to investigate this case and parents of children in the same school will be kept informed."
Roger Skilton, head teacher at Llangatwg, said letters had been sent to the parents of other pupils.
"The whole school community has been shocked and saddened by the sudden death of a Year 7 pupil," he said.
"She was a delightful young lady.
"She had only joined the school last September but had already shown herself to be a hardworking pupil with a very caring and considerate nature.
"She will greatly be missed by her many friends and all her teachers.
"Our heartfelt sympathies go out to her parents and family who will be devastated by their loss.
"I would ask for the privacy on the family, pupils and staff at the school to be respected during this very difficult time."
Ceinwen Williams, head teacher of Cilfrew Primary School where the girl had previously attended, said: "She has left the sweetest memories behind.
"She was the most beautiful and talented child in sport and drama.
"She was hardworking, diligent, warm-hearted, caring and wonderful with the younger children in the school."