The Centre for Health Protection and the Hospital Authority have started a joint investigation into the prevalence of an intestinal illness that killed a premature baby and made seven others ill in Queen Mary Hospital's neonatal intensive care ward. The seven babies were still being treated in an isolation unit and were in stable condition yesterday, according to the centre. The eight babies in the ward developed symptoms of necrotising enterocolitis between last Saturday and Friday. One of them died last Sunday. Medical experts say it is unclear what causes the illness - which mainly occurs in premature babies; however, they believe that bacteria from food can cause serious damage to already weakened stomach tissue. The centre began an investigation after the hospital notified it of the outbreak on Friday and closed the ward to new admissions. Staff from the centre yesterday visited the hospital to collect milk samples and samples from staff working on the ward for testing. Leung Pak-yin, the controller of the Centre for Health Protection, said: 'The hospital has carried out the necessary steps to control the infection ... They are now waiting for the findings of our investigation. The immunity of premature babies is usually very weak, so they become more susceptible to infection.' The chairman of the Hospital Authority, Leong Che-hung, said the investigation, headed by the centre, would look at milk supplies and needles as possible causes of the infections. All those possibilities are now under investigation,' he said. Permanent Secretary for Health, Welfare and Food Carrie Yau Tsang Ka-lai said it was 'very unusual' for eight babies to develop the disease at the same time.