Doctors are puzzled how swine flu, which is not considered highly lethal, could have killed a healthy three-year-old boy. They suspect that the child, who died in Tuen Mun Hospital on Wednesday, might have had other infections that aggravated his condition. But the Centre for Health Protection said it had found no other viruses or bacteria so far in specimens taken from the boy. Chinese University paediatrics professor Ellis Hon Kam-lun said that of at least 400,000 children infected with the swine flu virus last year, just one or two had died. 'So we don't see swine flu as life-threatening as people may fear,' Hon said. Last year, Prince of Wales Hospital admitted two children with severe swine flu symptoms who needed intensive care, but both recovered. One was a three-year-old girl who was found to be also infected with Streptococcus pyogenes, Hon said. But she was discharged from the intensive care unit a day after she was given antibiotics. She was discharged from hospital within a week. Hon said the dead boy could also have had other infections. Paediatrician Alfred Tam Yat-cheung agreed and said that whether patients with swine flu would die depended also on factors such as their immunity at the time and the amount of the virus they had contracted. Children and elderly people are usually more vulnerable because of their weaker immune systems. Tam, who is in private practice, said he had been seeing one or two children a day with swine flu in the past couple of weeks, a slight increase from previous weeks. The Centre for Health Protection said the dead boy's mother, who had also tested positive for swine flu, was discharged from Tuen Mun Hospital on Thursday night. His grandmother, who also had respiratory symptoms but did not have swine flu, was in stable condition in Baptist Hospital. Two pupils and a staff member at Sunkids Kindergarten in Tuen Mun, which the boy attended, had shown minor flu symptoms so far but did not need to be admitted to hospital, the centre said. The kindergarten was closed for seven days yesterday for disinfection. Teachers remained in school to carry out administrative work, according to one of the teachers, who refused to be named. Meanwhile, microbiology professor Paul Chan Kay-sheung of Chinese University said the flu peak had lasted for longer than usual this year. Chan said the peak season was usually in July and August, but there was no sign of it receding. He believed weather changes such as global warming might be a possible factor behind the change in flu patterns.