The world needs to be prepared for "unpredictable pandemics" from viruses making the leap from animals to people, scientists in Taiwan say.Their warning follows the first reported case of a common bird flu, H6N1, being detected in a woman, earlier this year.
The patient recovered and no other cases have been detected.
But the Lancet Respiratory Medicine report said "intensive" monitoring of bird flu was needed.
In May 2013, the first human case of an H6N1 bird flu was detected in a woman in Taiwan. One of her neighbours bred ducks, geese and chickens - although the precise source of the infection has not been detected.
Many sub-types of influenza, such as those that cause seasonal flu or the swine flu pandemic, are known to infect people, but H6N1 is not one of them.
The report, by the Centres for Disease Control in Taiwan, said: "The occurrence of a human case of H6N1 infection shows the unpredictability of influenza viruses.
"Our report highlights the need for influenza pandemic preparedness , including intensive surveillance for ever evolving avian influenza viruses."
Prof Wendy Barclay, from Imperial College London, said these infections may have happened in the past but improved technology had meant this one had been discovered.
She said: " Is this a truly new thing or are we now just better at seeing it?"
She told the BBC she expected far more of these cases to be reported in the next few years as more hospitals were geared up to look for novel bird flus.
Prof Barclay added: "This is a single case with no evidence of human transmission, but as always we should keep an eye on it and do studies to see how close it is to being able to spread between humans."