Almost 30 people have been struck down with an 'unidentified' form of pneumonia in China, sparking fears of another SARS epidemic.
Seven patients in Wuhan, Hubei province, are fighting for their lives in hospital from the mysterious disease.
Experts have been drafted in to investigate the cause of the disease, which officials fear is linked to Severe Acute Respiratory Syndrome (SARS).
The highly contagious virus, a form of pneumonia, killed hundreds of people in China and Hong Kong in the early 2000s. No cases have been recorded in the world since 2004.
The Wuhan Wuhan Municipal Health Commission today issued an emergency alert about the cases, which have all occurred since the start of December.
The organisation said hospitals across the city have treated a 'successive series of patients with unexplained pneumonia'.
Of the people infected, seven are in critical condition, 18 are stable and two are on the verge of being discharged soon.
All of the patients have been isolated and their close contacts are under medical observation, the Wuhan Municipal Health Commission said.
An investigation and cleanup were under way at a seafood market in the city where most of the patients worked.
It is suspected to be connected with the cases. Initial laboratory tests showed that the cases were viral pneumonia.
No obvious human-to-human transmission had been found and no medical staff had been infected, the commission said.
Experts from the National Health Commission is to carry out further tests, in hope of finding a cause of the cases.
An unnamed hospital source told the official People's Daily newspaper: 'The cause of the disease is not clear.
'We cannot confirm it is what's being spread online, that it is SARS virus. Other severe pneumonia is more likely.'
An official at Wuhan Central Hospital, where local media said some of the cases are being treated, declined to comment when contacted by Reuters.
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 World Health Organization (WHO) criticised China for under reporting the number of SARS cases following the outbreak.
SARS, which emerged in southern China in late 2002, spread rapidly from south China to other cities and countries in 2003. More than 8,000 people were infected and 775 died.
China sacked its then health minister Zhang Wenkang for the poor handling of the crisis in 2003, several months after the first case was reported.
WHO announced that China was free of the deadly SARS virus in May 2004.
Since 2004, there have not been any known cases of SARS reported anywhere in the world, the NHS says.
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