Chinese health authorities have not been able to identify a mysterious strain of pneumonia.

It has infected dozens of people and put the rest of Asia on alert — although they have ruled out a return of the deadly severe acute respiratory syndrome (SARS) virus.

A total of 59 cases of unknown viral pneumonia have been reported in the city of Wuhan, central China, with seven patients in critical condition, the Wuhan Municipal Health Commission said in a statement on Sunday. All patients are being treated in quarantine and no deaths have been reported.

According to the World Health Organization (WHO), symptoms of the pneumonia are mainly fever, with a number of patients having difficulty breathing, and chest radiographs showing invasive lesions of both lungs.

The outbreak came to light in late December and prompted fears in China of a possible resurgence of SARS, an acute viral respiratory illness first reported in the country in 2002 that caused a pandemic that ripped through Asia.

SARS spread to 37 countries worldwide, infected more than 8,000 people and killed 774 from November, 2002 to July, 2003. The illness is brought on by a coronavirus, and symptoms include fever, cough, severe headache, dizziness and other flu-like complaints.

Amid growing disquiet, Wuhan authorities said on Sunday they had excluded the possibility of SARS, Middle East respiratory syndrome (MERS), and bird flu.

If not SARS, then what is it?
According to the Wuhan health commission, the infection broke out between December 12 and December 29, with some of the patients employed at a seafood market in the city.

Local media reported that the market — which has been closed since January 1 for disinfection — also sold other live animals, including birds, rabbits and snakes, sparking concerns that the virus might have been transmitted to humans from animals.

Professor David Hui Shu-cheong, a respiratory expert at the Chinese University of Hong Kong, said it is highly likely that the outbreak was caused by a “brand-new viral pneumonia.”

“The concern now is whether the game (sold at the market) is one of the reasons for the outbreak,” he told CNN.

According to Hui, many new pathogens have been transmitted to humans from animals. The coronavirus that causes SARS was traced to the civet cat, a wild animal considered a delicacy in parts of southern China, where the epidemic first broke out. And dromedary camels are considered a likely source of MERS, he said.

Authorities have said that there has been no obvious evidence of human-to-human transmission so far, and no healthcare workers have been infected. At least 163 people who have had close contact with those infected have been placed under medical observation.

But there are still fears of a nationwide epidemic. The outbreak came just before the start of the busy Lunar New Year travel season, when hundreds of millions of Chinese are expected to be crammed into trains, buses and planes for family reunions. Millions of Chinese are also expected to travel overseas around Lunar New Year, which falls on January 25.

Professor Leo Poon, a virologist at Hong Kong University and a SARS expert, said the severity of the situation to come depends on whether the pneumonia in Wuhan can be transmitted among humans.

“If it only transmits from animals to humans, now that the market has been shut down and sanitized, the chances for people to be infected will be low,” he said.

Hui, from Chinese University, said the possibility of human-to-human transmission cannot be ruled out.

“Most often, respiratory viruses can be transmitted among humans. It is only a matter of how contagious it is,” he said.