A mysterious virus is making China (and the rest of Asia) nervous. It's not SARS, so what is it?

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.