Doctors have expressed concern after a virus carried in the brains of more than one-third of Hong Kong people killed an eight-year-old leukaemia sufferer. The girl died last year in the Prince of Wales Hospital - the first reported case worldwide of herpes virus 7 (HHV-7), an apparently mild virus, causing death in humans. The virus triggered fatal encephalitis, or inflammation of the brain. Chinese University microbiology professor Dr Paul Chan Kay-sheung, who heads a research team studying the virus, said that even though this was the only confirmed case and it affected a girl whose immunity system was depleted by leukaemia, he also saw about 10 other patients a year whose encephalitis was triggered by unknown causes. He suspected those cases also could be related to HHV-7. Dr Chan sought to ease any concerns among the public, saying there was no evidence the virus threatened healthy people. He called for more research. The young leukaemia patient developed her encephalitis about a month after she received a bone-marrow transplant at the university hospital. Dr Chan said it was HHV-7 that caused the condition that took her life. He said hospital doctors were shocked that the virus had 'activated speedily and fiercely', killing the victim within days. 'For the first time, we confirmed HHV-7 could cause encephalitis and could lead to death,' he said. Dr Chan said HHV-7 was common, with more than half of toddlers contracting it between the age of six months and two. More than 90 per cent of adults are found with the virus. Most carriers have no symptoms but the virus can cause minor irritations such as fever and rashes for about a week among children. Dr Chan estimated 36 per cent of the SAR population had the virus in their brains, based on a medical study last year of 300 brain tissue samples. A further 54 per cent carried it in other organs. 'Our concern is the virus is so common and stays permanently in human bodies . . . . We do not know when it will activate and develop encephalitis or other fatal diseases,' he said. HHV-7 and HHV-2, a common sexually transmitted disease, belong to the same virus family, but HHV-7 is transmitted through saliva and is difficult to prevent. Dr Chan said HHV-7 was believed to have existed in humans for centuries but studies did not begin until 1989. Many questions remain unresolved, such as whether the virus can trigger other fatal diseases or in which other parts of the body it can hide. 'This is only the first step in our study of the virus. We are appealing for more government funding to continue our research, especially to find treatment for future cases,' Dr Chan said.