Hong Kong has reported its first case of the H5N1 avian influenza virus in 18 months, putting the country on heightened alert.

Health authorities said that a two-year-old mainland child tested positive for the virus and is in serious condition in a Hong Kong hospital. Tests have been ordered for about 80 people who came into contact with the boy, who lives in the southern Chinese city of Guangzhou. He is said to have visited a live poultry market in the last month.