Hong Kong health officials raised avian influenza alert levels to serious late Monday after a domestic worker in the city became critically ill with the virus, the first reported case in the territory.

The 36-year-old Indonesian woman may have contracted the deadly virus after traveling across the mainland border to Shenzhen, where she purchased a chicken, slaughtered it and then ate it, health officials said. Exposure to live poultry is believed to be a major source of transmission of the H7N9 virus, which has killed 45 people in eastern China since it was detected earlier this year.

But the virus’s spread to Hong Kong is alarming given the city’s status as an international transportation hub and its history as the epicenter of the SARS epidemic, which quickly spread through the territory in 2003, killing 299 people and leaving 1,800 others extremely ill.

The woman who became ill with avian influenza was not identified; she "is now in critical condition at Queen Mary Hospital," said Dr. Ko Wing-man, Hong Kong’s secretary for food and health. Officials said that four people in the home where the stricken woman worked — the two parents and two children — showed possible signs of illness.

Health officials said they would tighten infection controls at hospitals and limit visiting hours, while strictly enforcing cleaning rules at live chicken stalls in local markets.

Hong Kong also plans to step up health checks at its borders to screen visitors for the virus, the territory’s chief executive, Leung Chun-ying, said today.

© 2013 The New York Times Company