A new and potentially deadly form of bird flu has claimed its first confirmed human life.
Tests revealed that a previously-unknown substrain of the H10N8 virus killed a woman who was admitted to hospital in China with fever and pneumonia.
The woman, from Nanchang City in Jiangxi province, died nine days after becoming ill despite antibiotic and antiviral treatment.
Experts believe the strain spread from poultry and may pose a pandemic threat to the human population.
The dead woman had visited a live poultry market a few days prior to infection, suggesting an incubation time of around four days - similar to that of other bird flu strains.
Reports suggest the victim was not an isolated case. At least one other person is believed to have been infected by the same strain in Jiangxi Province.
Dr Yuelong Shu, from the Chinese Centre for Disease Control and Prevention in Beijing, said: "A genetic analysis of the H10N8 virus shows a virus that is distinct from previously-reported H10N8 viruses, having evolved some genetic characteristics that may allow it to replicate efficiently in humans."
The strain is thought to have emerged from multiple re-assortments of genes from different bird flu viruses.
Notably, it shares genes with three other bird flu strains, H9N2, H7N9 and H5N1, the last two of which have spread to humans.
Scientists conducted tests on swab samples taken from the woman victim's windpipe.
Their results, reporting the first human death associated with H10N8, appear in The Lancet medical journal.
Co-investigator Dr Qi Jin, from the Chinese Academy of Medical Sciences and Peking Union Medical College in Beijing, said: "Importantly, the virus had a mutation in the PB2 gene that is believed to be associated with increased virulence and adaption in mammals, and could enable the virus to become more infectious to people."
The H10N8 strain was previously isolated from a water sample taken from Dongting Lake in Hunan Province in 2007. In 2012, it was detected at a live poultry market in Guangdong Province.
Co-author Dr Mingbin Liu, from Nanchang City Centre for Disease Control and Prevention warned that the threat should not be taken lightly.
"A second case of H10N8 was identified in Jiangxi Province, China on January 26 2014," said Dr Liu. "This is of great concern because it reveals that the H10N8 virus has continued to circulate and may cause more human infections in future."
Dr John McCauley, director of the World Health Organisation Collaborating Centre for Influenza at the Medical Research Council National Institute for Medical Research (NIMR) in London, said: "This case reminds us to be aware of human infections from animal influenza viruses, like the H7N9 cases in China which increase daily. Previously we did not think that H7N9 infections might be so lethal. Now we also must consider H10N8 infections as well.
" More human infections by avian H10N8 viruses cannot be ruled out. H10N8 viruses are of low pathogenicity in poultry and so infection in birds is not easy to detect. Whether humans are frequently exposed and infected is also not known. "
He added that the underlying condition of the dead woman was likely to have worsened her infection.
Dr Steven Riley, reader in infectious disease ecology and epidemiology at Imperial College London, said: " Although many more people are tested for influenza now than before, this first case of H10N8 is still a significant event."
Dr Jeremy Farrar, director of the Wellcome Trust, the UK's biggest research charity, said: " We should always be worried when viruses cross the species barrier from birds or animals to humans as it is very unlikely that we will have prior immunity to protect us.
"We should be especially worried when those viruses show characteristics that suggest they have the capacity to replicate easily or to be virulent or resistant to drugs. This virus ticks several of these boxes and therefore is a cause for concern.
"We are now much better at surveillance and at sharing information on emerging infections. It is highly unlikely this event would even have been noticed or reported just a few years ago. But we need to work out how we deal with this new information."
Virologist Dr Ben Neuman, from the University of Reading, made an appeal for calm. He said: " This fatal case of the flu is a personal tragedy for the family and friends of the victim.
"While the H10N8 virus bears watching, there is no cause for alarm at this time. The patient will have had contact with tens or hundreds of friends, family, acquaintances and health care workers before the severity of the disease was apparent. As yet there are no reports that the virus has spread to anyone else."