Image caption Postive tests on a dead chicken last week prompted Hong Kong's government to issue an alert

A man who had been diagnosed with China's first case of bird flu in more than a year has died in the southern city of Shenzhen, health officials say.

The 39-year-old bus driver was admitted to hospital with pneumonia but tested positive for the bird flu virus.

The H5N1 bird flu strain has a high level of mortality, killing up to 60% of humans infected with it.

Positive tests on a dead market chicken last week prompted nearby Hong Kong's government to issue an alert.

Hong Kong authorities culled 17,000 chickens after three birds were found to have died from the H5N1 bird flu strain.

It also banned imports and the sale of live chickens for three weeks after the infected chicken carcass was found at a wholesale market.

Analysis Hong Kong's Chief Executive Donald Tsang was quick to reassure the Hong Kong public, following the bird flu death just north of here in Shenzhen. He emphasised how good the health notification system was between mainland China and Hong Kong. The city is particularly fearful of pandemics, not only due to its dense population housed largely in high-rise buildings, but because of the city's health history. In November 2010, a 59 year old woman was diagnosed with the H5N1 avian flu virus, the first case in the city for seven years. She had been travelling in mainland China and survived. But in 1996, Hong Kong was the site of the world's first cases of mutated bird flu affecting humans. Six people died. In 2003, nearly 300 people died from Severe Acute Respiratory Syndrome (SARS).

But it was not clear whether the chicken came from a local farm or was imported.

The Shenzhen victim had not been in contact with poultry, nor travelled recently, China's Ministry of Health told Hong Kong health authorities.

In November 2010, a 59-year-old woman was isolated in Hong Kong with bird flu but survived.

In October 2011 a 29-year-old woman confirmed to have contracted the virus died on the Indonesian island of Bali.

The World Health Organization says bird flu has killed 332 people since 2003.

The virus has been eliminated from most of the 63 countries infected at its 2006 peak, which saw 4,000 outbreaks across the globe, but remains endemic in Bangladesh, China, Egypt, India, Indonesia and Vietnam.

China's Ministry of Agriculture warned last month that the bird flu virus seemed to exist widely in the poultry markets of mainland China, particularly in the south.

Research fears

BIRD FLU BASICS There have been more than 565 cases of human infection with the H5N1 virus since 2003

According to WHO figures updated in October 2011, 332 people have died of the virus

Most cases have been in south-east Asia and associated with close contact with diseased birds

Birds shed the virus in their faeces so close contact with their environment can spread disease.

The virus could potentially spread from person to person Q&A: Bird flu

Meanwhile, the World Health Organization has expressed deep concern about the way research was being carried out on the H5N1 virus, which can be fatal if transmitted to humans.

Such work carried significant risks and must be tightly controlled, said the WHO.

Scientists in the Netherlands and the US said last week they had discovered ways in which the virus might mutate so it can spread more easily to - and between - humans and other mammals.

The US government has asked the scientists not to publish full details, in case the information is used to produce a biological weapon.