Comparisons to Severe Acute Respiratory Syndrome (SARS) were drawn on social media after the city's health department posted a notice online Monday which instructed hospitals to report any and all new cases of the as yet unidentified viral pneumonia.
The organisation said hospitals across the city have treated a "successive series of patients with unexplained pneumonia".
At its height, SARS infected more than 8000 people worldwide, killing 349 in mainland China and a further 299 in Hong Kong between 2002 and 2003.
Clean-up efforts at a seafood market where some victims were vendors have been completed, the city officials said, adding that no obvious human-to-human transmission had been seen and no medical staff had been infected.
The patient sought treatment for symptoms, which include fever, at Tuen Mun Hospital in Hong Kong's New Territories on December 31, said the Hospital Authority, which oversees the Chinese city's public medical institutions.
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Of the people infected, seven are in critical condition, 18 are stable and two are on the verge of being discharged soon. Most of the patients worked in a seafood wholesale market, the commission said in a separate statement.
Several people were arrested for circulating fake news online about the viral spread of pneumonia, provincial authorities said, adding that rumors on social media alleging that there had been an outbreak of SARS are untrue, and no person-to-person transmission has been found so far.
On Friday, Singapore's health ministry said it would begin temperature screening on passengers arriving on flights from Wuhan.
An unnamed hospital source told the official People's Daily newspaper: 'The cause of the disease is not clear.
The MOH has also alerted all medical practitioners to be vigilant to look out for suspected cases with pneumonia who have recently returned from Wuhan.
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'We can not confirm it is what's being spread online, that it is SARS virus.
A World Health Organization (WHO) official told the South China Morning Post (SCMP) that the group is in contact with Chinese authorities and that it is closely monitoring developments.
In 2003, Chinese officials covered up a SARS outbreak for weeks before a growing death toll and rumours forced the government to reveal the epidemic.
The sickness, which is being compared to bird flu and SARS, appears to have originated in or near the Huanan seafood market, and some of the original infections were traced to workers who operate stalls there.
Chinese state media outlets have stressed that the cause of the current illnesses remains unknown, that fears of a new SARS epidemic are premature, and that the country has a robust system for dealing with epidemics.
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