AMNA NAWAZ: A highly contagious avian flu is infecting birds all over the globe.

Here in the U.S., farmers have lost or had to kill over 58 million birds to try and prevent the virus' spread.

So far, this flu hasn't caused any serious threat to humans.

But as William Brangham reports, there are several new outbreaks that are raising some alarm.

WILLIAM BRANGHAM: Thanks, Amna.

Over the last two years, the spread of this strain, known as H5N1, has been largely limited to birds.

But now two particular outbreaks, one among farmed mink in Spain and another among wild sea lions in Peru, suggests that H5N1 might now be able to spread between mammals.

And that's raised concerns about the virus' potential to spread further and perhaps even make humans sick.

Scott Hensley is a professor of microbiology and an influenza researcher at the University of Pennsylvania.

Scott, thank you so much for being here.

Before we get to the mink and the sea lions, can you just remind us how bad H5N1 is among birds all over the world right now?

SCOTT HENSLEY, Penn Institute for Immunology: We have seen H5N1 circulate in birds before.

What makes this current situation unique is how widespread this particular clade of H5N1 virus is spreading.

It's not a good time to be a bird today, because this virus has infected wild bird populations.

And it's also infected domestic birds all across the world.

And, again, the unique thing about this particular virus is, it is getting into bird populations that we have not seen be infected historically with H5 viruses in the past.

WILLIAM BRANGHAM: Yes, I was actually at an avian center in Minnesota several months ago, and we're seeing eagles and owls and raptors of all kinds getting this virus.

But, with regards to this -- these cases of the sea lions and the mink, those seem to suggest that the virus is now spreading within mammals.

Do you think that that evidence is -- do you think that that's what's happening?

And, if so, why is that a problem?

SCOTT HENSLEY: Yes, so that's exactly what makes us a little bit nervous.

So, these viruses are obviously very good at attaching to avian cells and getting into bird cells and replicating.

We know that because the virus is spreading again very rapidly among birds.

The good news is, is, the virus doesn't appear to be able to replicate in human cells very effectively.

But we have seen cases now, as you just outlined, the virus getting into mink, and there's likely mink-to-mink transmission that have occurred in the situation that we have seen in Spain.

And we see the virus spreading to other mammals as well.

This is alarming, because what we're afraid is that the virus might start changing.

It might undergo acquiring different substitutions that enable better replication in these mammals.

And we're afraid that some of those same substitutions might enable the virus then to attach to human cells more effectively.

Right now, again, the good news is, the virus appears to be very poor at infecting human cells.

But we start to get nervous when we see these essential crossover events, when these avian viruses are getting into other animals.

WILLIAM BRANGHAM: There have been, as I have mentioned briefly, some cases where the virus has jumped in, mostly when -- I think of this one case in Colorado where a person was cleaning out a chicken farm that had suffered a big outbreak.

And so that wasn't a human-to-human transmission.

It was just someone getting a big dose of this virus.

What do we know about what this virus does to people if it does get into people?

SCOTT HENSLEY: Well, there -- luckily, there's been very limited number of infections with this particular H5N1 virus.

In the past, H5N1 viruses have been shown to have very high mortality rates.

But, again, in this current outbreak, we have seen a limited number of human infections, mostly with folks who have had very close contact with birds.

And there has not been many serious infections to date.

So, H5N1 certainly has the potential to cause a lot of disease and high mortality rates.

This particular virus does not seem well-adapted at infecting humans.

But this could change, of course, and something that we have to keep our eye on.

WILLIAM BRANGHAM: So can you help put this into perspective for people?

Those people who might be seeing these reports and being alarmed about this idea of it jumping into humans, how worried should we be?

SCOTT HENSLEY: Look, we don't have to be alarmed right now.

Look, don't run out to your pharmacy and buy 1,000 rolls of toilet paper.

(LAUGHTER) SCOTT HENSLEY: If you see a dead bird or a sick bird, certainly, stay away from it.

It's likely that this virus needs to acquire several substitutions before it can efficiently spread from human to human and start infecting humans.

But we need to remain vigilant.

The research community right now has to come together.

We need to increase funding for surveillance to be able to track this virus in real time.

And we have to understand better what changes this virus needs to have take place for it to jump into the human population.

So, an everyday person right now, again, avoid sick birds.

I think, other than that, there's not a whole lot that you need to do.

But this is the time for the research community to really come together and increase our surveillance efforts and start developing new vaccines in case this virus does jump into humans.

WILLIAM BRANGHAM: All right, that is Scott Hensley at the University of Pennsylvania.

Thank you so much.

SCOTT HENSLEY: OK.

Thank you for having me.