Officials activated a new “serious response” level, which indicates a moderate impact on Hong Kong’s 7.5 million people, The Associated Press said. It is the second highest in a three-tier system that is part of a new government plan launched Saturday to respond to infectious diseases of unknown cause.

The outbreak started last month in the mainland city of Wuhan, a city about 600 miles northwest of Hong Kong. At least 44 people have fallen ill, with 11 of them in critical condition, the South China Morning Post reported.

All were being treated in isolation and 121 others who had been in close contact with them were under observation. So far, no medical workers have been infected and there have been no clear indications of human-to-human transmission of the disease.

Many of the ill worked at a seafood market that also sold live wild animals like rabbits and birds that can carry viruses dangerous to humans. Avian flu has been ruled out, along with influenza.

While the market has been scoured, the outbreak has prompted worries about the return of the 2002-2003 SARS epidemic that started in southern China and killed nearly 800 people and infected more than 8,000 in the mainland, Hong Kong and elsewhere.

The most common symptom is fever, with shortness of breath and lung infections in a small number of cases, the Wuhan Health Commission said.

Hong Kong’s health department added an additional thermal imaging system at the city’s airport to check the body temperature of arriving passengers. Temperature checks are also being done at the West Kowloon high-speed rail station that connects Hong Kong to the mainland.

Screening measures were also adopted in Singapore and Taiwan.

The World Health Organization is “closely monitoring” the outbreak.