The number of cases of a highly contagious and potentially fatal childhood disease has hit the highest level since records began and experts have no idea why.
A total of 272 scarlet fever cases have been reported so far this year, including one in which a seven-year-old girl died on May 29 - the first fatal case in five years.
There was speculation that a 15 year-old boy who died in Princess Margaret Hospital on Tuesday was a victim of the disease, but initial tests appear to rule this out.
Since scarlet fever was made a notifiable disease in 1997, the previous highest number of annual cases was 235 in 2008.
There were 187 cases in 2009 and 128 last year.
Food and Health secretary Dr York Chow Yat-ngok said more studies were needed to find out why there were so many cases this year.
'Infectious disease infections fluctuate every year. Perhaps it was related to the level of antibodies present in the general population. When it's lower, more people will be infected,' he said, adding that the situation was not that alarming as there had only been one death.
University of Hong Kong microbiologist Professor Ho Pak-leung said there were usually more cases in the first half of a year. He said 140 of 187 cases in 2009 were in the first six months.
Scarlet fever is an infection caused by bacteria and can spread through sneezing, coughing or physical contact with respiratory secretions. It normally affects young children aged two to eight, causing a fever, sore throat and rash.