Anthony Fauci, a director at the National Institutes of Health, talks about the threat of H1N1, and what health officials are watching for in the future.

Dr. Anthony Fauci:

When people have asked me over the years what I was most concerned about from an infectious disease standpoint, I have consistently said from 20 years ago, 15 years, 10 years ago would be an influenza pandemic that had both great transmissibility capabilities as well virulence. I still am concerned that someday that will happen to us. I think we're somewhat fortunate now.

It’s unfortunate that we’re in the middle of a pandemic, but it’s somewhat fortunate that despite the great transmissibility of this particular virus, it is still relatively a mild pandemic, mild to moderate.

We don't really know why some individuals who are otherwise perfectly healthy can actually get very sick with influenza and die. If, if you look at the deaths in the H1N1, about 70 percent of them are associated with people who have an underlying condition that would predispose them to getting complications from influenza. But that leaves about 30 percent of people who for all other practical purposes, are really quite well and robust.

We don't know why that happens, but it is not surprising because there's always a great degree of biological variability. So when you have a species like the human species, even people who are really quite well, the vast majority of people will do well but there are some people who will do very poorly and some people who won't even notice that they're sick.

And I think what you're seeing, when you see that 30 percent who are otherwise well and healthy, who get serious disease, I think that's part of the bell-shaped curve of human biology and nothing more then that.

The H1N1 pandemic flu is very, very good at spreading from person to person.

So it's a very good transmitter, so we know that. Thus far, after months and months of circulation, it does not appear to have increased in its virulence. So although in a very, very small rare situation, it can cause serious illness and death, particularly in young people. For the most part, it's the relatively mild to moderate influenza.

It has not genetically changed, we first noticed it in April in the United States and in Mexico. It's been all around the world now as we are coming into our own flu season and it really hasn't changed much at all with any significant mutations. So that's encouraging news.

Influenzas have the track record of being quite unpredictable and that's the reason why we monitor it very carefully cause you never know what's going to happen with influenzas.

Once the virus hangs around society for awhile and most of the people are relatively immune to it, then they're going to into a person in which the immune system is going to start fighting it and it's going to force that virus to mutate to escape the immune system. Then that virus will infect another person and it'll force it to change a little bit more.

And then before you know it, you have a bit of a different virus that originated with the one you started with but mutated enough now that it's really a different virus.