Three children were admitted to Whanganui Hospital with chickenpox over the weekend.
Five-year-old Isaac Mahony was "shaking, having trouble breathing and walking, and fighting a temperature up in the 40s" when his parents took him to the emergency department, mum Malissa Mahony said.
Isaac caught chickenpox after an outbreak at Upokongaro School, several days after his parents received a newsletter warning parents the disease was going around.
"Five days after Isaac became unwell, my husband and I took him to the Whanganui Hospital's emergency department," Mrs Mahony said.
Paediatrician Giles Bates said two of the three children admitted on the weekend needed intravenous antibiotics to battle infections caused by the disease.