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.
A new and deadly form of bird flu has claimed its first confirmed human life.
The previously-unknown substrain of the H10N8 virus killed a woman who was admitted to hospital in China with fever and pneumonia, test results revealed.
The 73-year-old woman, from Nanchang City in Jiangxi province, died nine days after becoming ill despite antibiotic and antiviral treatment. She 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.
The strain is believed to have spread from poultry and may pose a pandemic threat to the human population, experts said.
Reports suggest at least one other person is critically ill in hospital after being infected by the same strain in Jiangxi Province.
The World Health Organisation's (WHO) top influenza expert, Dr. Keiji Fukuda, said that people appear to catch the latest 'bird flu' virus, H7N9, from birds more easily than the H5N1 strain that began ravaging poultry across Asia in 2003.
Health experts are concerned about H7N9's ability to jump to humans, as well as the strain's capacity to infect birds without causing noticeable symptoms - making it difficult to monitor its spread.
"This is definitely one of the most lethal influenza viruses we have seen so far," Dr Fukuda said. But he added that experts are still trying to understand the virus, and that there might be a large number of mild infections that are going undetected.
The H7N9 bird flu virus has infected more than 100 people in China, seriously sickening most of them and killing more than 20, mostly near the eastern coast around Shanghai.