GEOFF BENNETT: The H5N1 virus, also known as bird flu, is spreading rapidly among dairy cows.

It's been found in over 85 herds across 12 states.

At least three farmworkers have now been sickened by the virus.

And while public health officials say the risks to humans remain low, concerns are growing.

William Brangham has more.

WILLIAM BRANGHAM: So, how serious is this threat?

And are we doing enough to keep our eyes on this evolving virus?

To answer that, we are joined by Rick Bright.

He's the former head of BARDA, which is the U.S. government's lead agency for developing countermeasures for public health emergencies.

Rick Bright, thank you so much for being here.

You have called this a -- quote -- "dangerous inflection point" that we are in right now.

And you recently wrote -- quote -- "that if we keep ignoring the warning signs, we have only ourselves to blame."

What are the warning signs that you're seeing right now?

And how are we ignoring them?

DR. RICK BRIGHT, Former Director, Biomedical Advanced Research and Development Authority: Well, William, thanks for bringing more attention to this crisis, this situation.

We're seeing more and more animals, mammals, in particular, infected with this virus, and, clearly, sustainability to infect dairy cattle and spread from cow to cow.

And we know that people are really in close contact with these mammals.

And the more mammals are infected, the more contact we have with them, the more risk we have of infecting people.

And we're starting to see that it happened over the last couple of months with three human cases.

WILLIAM BRANGHAM: So we have seen these several farmworkers that you mentioned.

The CDC mentioned that this most recent farmworker, the one in Michigan, exhibited somewhat different symptoms than the others had.

Why is that particularly troubling?

RICK BRIGHT: Well, it's really important to understand that, to spread this virus efficiently from person to person, it's going to have to get in the respiratory tract.

And this latest infection in this patient showed that the patient had respiratory symptoms, meaning he was coughing, for example.

The two prior cases only had an eye infection.

It's much harder to spread a virus from an eye infection than it is from a respiratory infection, when you're coughing.

And so that is really an important development causing great concern.

WILLIAM BRANGHAM: And we have not yet seen any transmission from a human to a human.

All these cases have thus far been one or more animals to one human, correct?

RICK BRIGHT: Well, William, that's where I'm really concerned.

When I talk about we're not doing enough about it is, we really don't know how many humans have been infected with this virus.

And, today, there's been very limited to almost no serology testing done among any of the cases, the close contacts on the farm or their family members.

So it's really hard for us to know how many people have been exposed, and if there really is any human-to-human transmission or not.

WILLIAM BRANGHAM: And how do you explain that?

Why is it -- given that there are certainly enough researchers like yourself within the federal government who understand the potential here, why isn't that we're not doing more testing?

RICK BRIGHT: Well, we're finding that it's not always a case of not having enough tests in this situation.

But we have tests at the CDC, for example.

But the problem is getting access to the workers to test them.

So there are a number of issues and barriers that we're encountering.

And some of those have to do with the work force itself.

A large part of the work force on the dairy farms are undocumented immigrants.

And there's concern among those workers about being tracked or traced by the government.

And they might have to face immigration issues.

There are also concerns of the farmers and the people who own the farms, if the federal government or even state government were to come onto the farm, find a widespread outbreak in animals or people, have to shut down the farm, perhaps lose their work force and lose their income.

And there's not enough incentives to really break through those barriers right now.

WILLIAM BRANGHAM: And what would those incentives be?

I mean, I understand the farmer not wanting to have a scarlet letter put on their herd or on their farm.

And I understand what you're saying about the workers as well.

But how do we incentivize people to do the right thing to keep us all safe?

RICK BRIGHT: We're going to have to put some measures in place to ensure that the workers who get tested have the right care that they need, the right health care, because many of them don't have health insurance.

If they lose work because they're sick and we want them to not go back into that environment, then there should be compensation for sick leave and for those lost wages.

If a farmer has to put down a cow or keep more cows out of the commission longer, instead of putting them back on the milking line while they might still be infected, then we have to compensate the farmers for the loss of that cow or, for the loss of that milk production.

WILLIAM BRANGHAM: So the government says that the milk supply is safe, that pasteurization kills the virus.

They say that the human cases we have seen don't indicate that the threat to us as humans is severe.

How worried are you that this could become the next pandemic?

RICK BRIGHT: Well, I'm more worried right now, William, about the information and the data that we're missing than the data that we have.

So if you just looked on the surface - - and that's really all we're seeing right now -- there may not be too many immediate flares of concern of a forest fire or so.

But if you look beneath that surface, if we do more testing, we might see that this virus is changing in remarkable ways that would give it an advantage if it were to really infect people and transmit easily person to person.

Since we're being blindfolded in this battle right now, I'm really concerned that the virus is winning the game and getting ahead of us.

WILLIAM BRANGHAM: All right, Rick Bright, the CEO of Bright Global Health, thank you so much for being here.

RICK BRIGHT: Thank you, William.