Virologist Dr Chris Smith, who is based at Cambridge University, made the comments after two poultry workers in the UK tested positive for the virus.
"This is probably the biggest outbreak of bird flu that the world has ever seen - not amongst the humans but amongst the bird population obviously and it has been going on, flying under the radar for a couple of years," he told TalkTV on Wednesday.
"It has largely been eclipsed by our worries about COVID, it didn't get the air time that otherwise it would have deserved.
"More than a billion birds around the world are thought to have died or been culled because of this new emerging spreading strain of flu.
The virus currently has a 50-70% mortality rate in humans, though if it did start spreading widely in the human population, it is highly likely this rate would significantly decline.
Dr Smith added: "Is that a big threat? Well, any emerging disease is a big threat because it could jump the species barrier... periodically you do get these incursions of bird viruses into people and whenever the virus jumps a species barrier it has an opportunity to evolve further to become better at spreading in people - there's no evidence at this state that it's doing that though."
The two workers recently affected in the UK are now testing negative for the virus, and did not have any symptoms when they tested positive, according to the UK Health Security Agency (UKHSA).
"Bird flu H5N1 when it infects a person can have a mortality rate as high as 70%," explained Dr Smith. "This is actually quite reassuring news that it has been detected. Why, because it shows that the screening programme is working. There's enhanced screening going on to keep an eye on people working closely with birds to see if they are remaining safe."
In the UK, the virus was previously detected in one other human being in January 2022, when a duck keeper contracted the virus from his infected birds. He later tested negative for the virus and recovered.
Can humans catch bird flu?
According to the NHS, humans can get bird flu by coming into close contact with
birds - including from touching infected birds or their droppings or bedding, as
well as from killing or preparing infected poultry to cook. Additionally, the NHS
warns, poultry markets can be a source of bird flu.
You cannot get bird flu from eating eggs or fully cooked poultry, it adds.
Given the high mortality rate of the virus, host Julia Hartley-Brewer questioned why lockdowns hadn't been imposed given the relatively low mortality rate in COVD.
"It's a numbers thing," Dr Smith replied. "With COVID pretty much the entire world population was susceptible and we think that most of the world's population has now had at least one brush with that virus... that is not the case with the numbers with bird flu.
"Usually as viruses adapt to people and they become better bedfellows with us, they are better at infecting us, spreading among us, they surrender some of that mortality, some of that virulence in order to be more spreadable so we would expect that as the flu humanised it would become less lethal."