H5N1 has made the jump to cattle, showing up in herds across the US in recent weeks. Scientists believe the virus might be passing from cow to cow, not relying on transmission from infected birds. There are reports of cats on farms getting infected after drinking cows milk, and one case of a human in the US contracting the disease after contact with cattle.
Dr Richard Webby is an infectious diseases researcher at Saint Jude's Children's Research Hospital in Memphis, Tennessee, and the director of the World Health Organisation's Collaborating Centre for Influenza Studies.
He told RNZ's Sunday Morning programme the virus' sudden jump into cows and its fast spread took experts by surprise.
"It's almost like the Crusaders losing five in a row. Had you asked me six weeks ago what the chance [was] of finding this virus in cows, I would have said none. So it's a completely new event for us.
"We think we know this virus, and then something happens next week that teaches us that we didn't really know anything about it."
There have been hundreds of human H5N1 cases reported since the virus was discovered in the late 1990s - about half of those fatal, making it potentially far deadlier than Covid-19, even before we had vaccines.
A recent international survey of senior disease experts revealed 57 percent think a strain of flu will be the cause of the next global outbreak of deadly infectious illness.
"Influenza is always at the top of the list," Webby said. "We don't need to know exactly what the next agent is going to be, but you know, preparing for one may very well give us a leg up [in] preparing for the other."
While humans can be infected with H5N1, there is only limited evidence of human-to-human transmission.
"It still would rather be inside a bird replicating than anywhere else," Webby said. "But any time it does get into a mammal, that at least theoretically gives that virus a little bit more opportunity to change and perhaps be a little bit more mammal, like a little bit more able to infect mammals, including humans.
"So, from that perspective, any time we see it in a mammal it's of concern, but we're now going into this a little bit blind, to be honest with you - we don't know what is likely going to happen here. We don't know what the virus is going to do in terms of changing."
If it started infecting pigs, that would be cause for greater concern.
"They have their own flus. A lot of them are actually viruses that we humans have given them and they've managed to be maintained.
"But when we look at what we do know about the different hosts, we do know that when these avian viruses, if they get into pigs, then growth in the pig can actually make those viruses more infectious for humans. So, yeah, that's why we absolutely don't want this virus in that host."
It it unlikely we will get any advance warning from scientists if it did start spreading between people, he warned.
"If all of a sudden the virologic stars align and what's a bird virus in a cow one day suddenly gets all of the mutations it needs to be a human virus, then the chances of us picking that up in advance are pretty slim. We're going to find that when we start to see accumulation of human cases."
As for New Zealand's sizeable dairy industry, Webb said the risk - for now - was low.
"I think if I was a cow in New Zealand, to be honest, I'd be feeling pretty safe from this virus. If I was a chicken, maybe I'd be feeling a little less safe. But what I mean by that is this episode in cows, this is the first time we've even seen it over the 25-odd years that we've been tracing.
"The great, great, great, great, great granddaddy and grandmothers of this virus started in Southeast Asia back in the late 1990s. We haven't seen it in cows before. So, even though this virus has been there for that long, it's transitioned to cows, it's been pretty rare.
"So, put yourself in a New Zealand situation. If you're a cow in New Zealand, it's not just the fact that this virus doesn't seem to get into cows very much, but you don't even have the virus there yet. So it's a pretty safe place to be."
The Ministry for Primary Industries and Department of Conservation have been preparing a contingency plan for an inevitable "tsunami" of bird flu coming to New Zealand. It has already been found in Antarctica.
There are some migrating birds that visit New Zealand.
"Just because New Zealand doesn't have it yet, I don't think, I wouldn't say that's going to be a true statement forever. But the chance of it, one, getting into New Zealand and then two, getting into a New Zealand cow, I think that the chances are pretty slim."
Coming so soon after the world-changing Covid-19 pandemic, the biggest of its kind in a century, Webby feared the public would not take the threat seriously. SARS-CoV-2 - the virus behind Covid-19 - made the leap to humans from an animal host sometime in late 2019, scientists believe.
"This is a bad time for this to be happening - 'It's those scientists again, they're going to worry us about a virus that's going to come and kill us all tomorrow:'...People don't want to hear about this.
"They don't want to hear about being vaccinated against a new virus again. So yeah, [the] timing is not good, for sure."
If an outbreak in humans did happen, Webby also feared officials - despite lessons learned from Covid-19 - would not be ready.
"It may not be this virus, but something is absolutely going to do that. It's going to jump from an animal into humans and we're going to be Covid pandemic all over again, or it could be Covid pandemic sort of supercharged as well.
"There's clearly not enough resources going into this preparedness of pandemics, and that's not a New Zealand-centric issue, it's not a US-centric issue, it's a global issue. You know, it's almost like we're not willing to sort of put those resources into the insurance we really need."
