Kiribati's Ministry of Health says rotavirus is responsible for the deaths of two children and around 500 cases of diarrhoea and vomiting.
Dr Teatoa Tira says eighty per cent of the cases are children under five years old and he says the two children who died were also suffering from malnutrition.
The Ministry is renewing calls for families to be careful with hygiene and to take any unwell children to hospitals or clinics.
He says the Ministry suspects the ground water supply is contaminated which happens at both times of heavy rain and during droughts such as the past month.
"We can't clean up the water supply. We just boil it. The water is there, we have no choice, that's the only source we have."
Dr Tira says climate change and overcrowding compound the problem of trying to manage illnesses like rotavirus.