Officials are warning of an outbreak of scarlet fever among children in Hong Kong.
There have been more than 400 cases of the disease this year, including the death of a six-year-old last month.
Initial tests suggest a five-year-old boy may also have died from the bacterial infection, which is spread by coughing and sneezing.
Scientists in Hong Kong believe the bacteria may be spreading more quickly than usual due to a genetic mutation.
Cases have also been seen in mainland China and Macau.
Dr Thomas Tsang, controller of Hong Kong's Centre for Health Protection, said: "If the genetic mutation is responsible for the increased transmissibility of the bacteria, the outbreak may be sustained for some time."
Scarlet fever happens every year in the region, but this year there have been more cases than usual.
Hong Kong has had 419 cases, already the highest annual total in the city.
Most are in children under 10, with clusters in kindergartens, primary schools and childcare centres.
Scarlet fever infections have gone up around fivefold in China, and threefold in Macau, about an hour by ferry from Hong Kong.
According to a government statement, there may be regional outbreak of scarlet fever.