GEOFF BENNETT: There is growing concern tonight about the continuing spread of the highly contagious bird flu in the U.S. As William Brangham explains, while federal health officials say the risk to humans is low, the virus has now spread to dairy cattle and sickened one person -- William.

WILLIAM BRANGHAM: That's right, Geoff.

This strain of bird flu, called H5N1, has been sickening bird flocks across America for a few years now.

Millions have been killed to prevent further spread.

Yesterday, the country's biggest egg producer halted productions when chickens at one of its facilities got sick.

But this virus has also been infecting mammals, most recently dairy cows, in five different states.

This week, a person in Texas tested positive after working with cattle.

He's had mild symptoms and is expected to recover.

For more on this virus, we are joined again by Jennifer Nuzzo.

She runs the Pandemic Center at Brown University School of Public Health.

Jennifer Nuzzo, so good to have you back on the program.

Dr. Mandy Cohen, the head of the CDC, says the risk to humans from this virus is low, they're monitoring it, and there's no reason to worry at this point.

Is that where you come down?

DR. JENNIFER NUZZO, Brown University School of Public Health: So I think it's true that the risk to the general public is quite low, and I think it's also important that we continue to monitor this virus to make sure that doesn't change.

The one thing I do quite worry about is the risk to farmworkers, people who would be exposed to the sick animals, because we do know that exposure to sick animals can result in human infection.

And we have already seen that now to date in two cases in the United States, the most recent one being in a worker that worked with sick cows.

So I do worry about protecting farmworkers.

But for the general public, the risk of contracting this virus is currently low.

WILLIAM BRANGHAM: Let's talk about that infection.

So, this was someone who apparently had been working with cattle, cattle that had been infected with this bird virus.

How should we see that route of infection?

Because this does happen every now and then, a widespread infection amongst animals, and it jumps to one human.

DR. JENNIFER NUZZO: Yes, so we have been tracking this virus for 20 years.

And in the last two years, I would say, it has become a lot more concerning in terms of the number of species that this virus has shown itself capable of infecting.

We have historically called this a bird flu virus because it has predominantly affected wild birds and domesticated birds.

But over the last two years, we have really seen the species range that has been affected by this virus grow quite a bit, to include mammals.

So that's quite alarming.

This is also the first time we have seen it in cows.

Cows haven't frequently been seen to get influenza A, which is the particular type of influenza virus this one is.

So that is also somewhat new and surprising.

But we have in the past see humans that have had direct contact with sick animals get sick.

It doesn't happen all that frequently.

But when it does happen, it's quite concerning, because, in many of these cases, that infection has been quite deadly.

Fortunately, in the most recent case, the infection has been quite mild.

In fact, the symptoms are a little bit different than what we have typically seen in the past.

The sick worker had conjunctivitis and eye inflammation.

And so while, we're glad to see a mild infection, we don't yet understand why that is.

WILLIAM BRANGHAM: So, for now, at least, jumping from a group of animals to one human is one thing.

But if it were to go from human to human, that's when it would be really troubling; is that correct?

DR. JENNIFER NUZZO: The predominant reason that we're worried now is because we're worried that this virus may gain additional functionality and additional ability to infect humans, so that we could see human infections occur more frequently.

And the worst-case scenario that we worry about is this virus gaining the ability to be transmitted easily between humans.

In the past, there have been a few cases that we haven't been able to rule out limited human-to-human transmission, but that human transmission hasn't been sustained and it's been very rare relative to the number of human cases we have seen.

But we're watching this virus primarily to make sure it doesn't gain the ability to infect humans more easily and, worse, to be spread between humans easily.

WILLIAM BRANGHAM: And, right now, there is no evidence of that happening?

DR. JENNIFER NUZZO: No, there is not.

So we're not worried currently, but we don't often get head starts or sort of early warnings about future health threats.

And so why you're hearing a lot of scientists, experts and health officials talking quite concerned -- in a concerned manner about this particular development is because we want to make sure we get ahead of it.

We want to make sure we don't allow this virus to infect many more animals or many more humans to give it any ability to gain the functionality to infect humans more easily.

WILLIAM BRANGHAM: For people who are seeing that these are cases in dairy cows and egg-laying chickens and might worry, do I have to worry about milk and eggs, what do you tell them?

DR. JENNIFER NUZZO: Well, it's long been public health advice not to eat raw eggs or raw milk.

And that public health advice still applies.

As long as you're following that guidance, all the evidence we have so far is that you should be fine.

WILLIAM BRANGHAM: The U.S., as you well know, has been focusing on the threat of pandemic influenza for decades now.

But we saw how COVID punched all these holes in what we thought was a world-class, robust set of plans and preparations.

Are you concerned that if -- again, no evidence that this is happening now, but if this were to turn into something more severe, that we could respond in a smart and timely way?

DR. JENNIFER NUZZO: Well, I think that's why we're having this conversation now, is to talk about what triggers are going to make us act.

I think our recent experience with COVID was quite humbling in a lot of ways.

We have a bit more experience dealing with influenza and, in particular, dealing with a flu pandemic.

People may not remember that we actually had a flu pandemic in 2009.

That was not an H5N1 virus.

It was an H1N1 virus.

And in many ways, we do have more resources and more familiarity with flu.

So, in some ways, you could imagine it easier.

However, every type of event is going to be challenging.

So, I think that's why it's really urgent that we use this moment where we are getting some concerning signals from the animal world about potential future human health threats to make sure we have everything we need to be able to respond if this virus does take a turn for the worse.

WILLIAM BRANGHAM: All right, Jennifer Nuzzo, Brown University School of Public Health, always great to see you.

Thank you so much.

DR. JENNIFER NUZZO: Thanks for having me.