Homeland Security Secretary Janet Napolitano warned Wednesday that the threat of terrorism against the United States was in some ways "at its most heightened state" since the September 11, 2001 attacks.

In addition to the threats by al Qaeda, the militant group behind the attacks nearly a decade ago, Napolitano said the country faces new threats from those inspired by the group and those already inside the United States.

"The threat continues to evolve and in some ways the threat today may be at its most heightened state since the attacks nearly 10 years ago," Napolitano told the U.S. House of Representatives' Homeland Security Committee.

She also said in her testimony to lawmakers that U.S. officials believed there may be individuals who want to carry out attacks already in the country and that "they could carry out acts of violence with little or no warning."

Individuals associated with al Qaeda and the Taliban have tried to carry out several attacks against the United States, including by a Nigerian man who allegedly tried to blow up a U.S. airliner with a bomb hidden in his underwear and another individual who plotted to attack the New York subway system.

"As I have said before, we cannot guarantee that there will never be another terrorist attack, and we cannot seal our country under a glass dome," Napolitano said. "However, we continue to do everything we can to reduce the risk of terrorism in our nation."

The head of the National Counterterrorism Center, Michael Leiter, told the committee that the al Qaeda off-shoot based in Yemen, al Qaeda in the Arabian Peninsula (AQAP), at the moment represented the biggest threat to the United States.

Leiter said that the parent al Qaeda group, believed to be hiding in the Pakistan-Afghanistan border region, was probably at its weakest point since the September 11, 2001 attacks but remained a "very determined enemy."

"I actually consider al Qaeda in the Arabian Peninsula with (Anwar) al-Awlaki as a leader within that organization probably the most significant risk to the U.S. homeland," Leiter told the committee, noting that it has a large Internet following.

Al-Awlaki, a Muslim cleric who is U.S. citizen but left the country in 2001 and joined al Qaeda in Yemen, has been tied to plots against the United States over the last two years.

The group has claimed responsibility for the 2009 Christmas Day thwarted attack aboard a U.S. airliner and a more recent attempt to blow up two U.S.-bound cargo planes with toner cartridges packed with explosives.

Al-Awlaki also communicated with a U.S. Army Major Nidal Malik Hasan who in November 2009 allegedly went on a shooting rampage at Fort Hood, Texas, that killed 13 and wounded 32. Leiter said that it appeared to be more "inspiration rather than direction."

Davos, Switzerland (CNN) -- Microsoft founder Bill Gates sat down recently with CNN Chief Medical Correspondent Sanjay Gupta in Davos, Switzerland. The billionaire philanthropist was attending the World Economic Forum to push his mission of eradicating polio by 2012. Gates, through his foundation, also pledged $10 billion to provide vaccinations to children around the world within a decade.

Gupta asked Gates for his thoughts about the alleged autism-vaccine connection. He also asked: Who holds ultimate accountability for the billions of dollars being spent on aid? Is a certain amount of corruption and fraud expected? Below is an excerpt of their conversation.

Dr. Sanjay Gupta: Ten billion dollars [pledged] over the next 10 years to make it "the year of the vaccines." What does that mean exactly?

Bill Gates: Over this decade, we believe unbelievable progress can be made, in both inventing new vaccines and making sure they get out to all the children who need them. We could cut the number of children who die every year from about 9 million to half of that, if we have success on it. We have to do three things in parallel: Eradicate the few that fit that profile -- ringworm and polio; get the coverage up for the vaccines we have; and then invent the vaccines -- and we only need about six or seven more -- and then you would have all the tools to reduce childhood death, reduce population growth, and everything -- the stability, the environment -- benefits from that.

Gupta: There has been a lot of scrutiny of vaccines recently -- specifically childhood vaccines. There has been a lot of news about is there a connection with autism, for example. What do you make of all that? Dr. [Andrew] Wakefield wrote a paper about this [in The Lancet in 1998] saying he thought there was a connection. And there were lower vaccination rates over a period of time as a result in Britain, then the United States. What are your thoughts?

Gates: Well, Dr. Wakefield has been shown to have used absolutely fraudulent data. He had a financial interest in some lawsuits, he created a fake paper, the journal allowed it to run. All the other studies were done, showed no connection whatsoever again and again and again. So it's an absolute lie that has killed thousands of kids. Because the mothers who heard that lie, many of them didn't have their kids take either pertussis or measles vaccine, and their children are dead today. And so the people who go and engage in those anti-vaccine efforts -- you know, they, they kill children. It's a very sad thing, because these vaccines are important.

Gupta: Developing the vaccine. The scientific research that goes into that, obviously, is one thing, and then distributing things, even after they've been created, [is another]. Someone said to me once that even if the cure for HIV/AIDS came in the form of a clean glass of water, we still wouldn't get rid of AIDS in the world because of actually distributing some of these things. How do you address a challenge like that, no matter the money?

Gates: Well, there are fantastic ways of getting vaccines out -- a system that has been built up over the years. In the case of smallpox, they just used the vaccine and they eradicated the disease all the way back in 1979. We cover about 75% of the world's children with the vaccines. Vaccination is pretty special because you can do a vaccination campaign anywhere in the world. All you are doing is gathering women from the villages, getting them the vaccines and asking them to go around and find the children. And then you pay other people who are independent to come in, look at the children, survey, and see what the coverage rate is. You also have clear indicators. Measles will always show you if someone isn't doing a good job on vaccinations. Kids will start dying of measles. So we know, when we spend money, that the group we asked to do that vaccination, they've delivered.

Gupta: You talk about smallpox being a little bit of a model in terms of proof of principle that it can be done. D.A. Henderson -- Donald Henderson -- who was with the World Health Organization at the time this was done, has said look, when you talk about polio, is this more of a movement rather than a public health initiative using objective evidence. And I think what he was saying is that, should this be more about annual immunizations rather than trying to find this moment in time?

Gates: Well, when you talk to mothers whose children who are paralyzed, I think, no matter what you label it, it should be about getting rid of this evil disease. I don't think there is any philosophy that suggests having polio is a good thing. The world has been very careful to pick very few diseases for eradication, because it is very tough. After smallpox got finished, the lesson from that was the miracle of vaccines, not that we should immediately take on other diseases.

Gupta: You have talked about Afghanistan, Pakistan and the polio vaccine, and you've said that doing this, the vaccination campaign, can help stabilize a war-torn region like this.

Gates: What you are seeing is that the density in the poor areas is greater than they can grow the food, greater than they can educate, greater than they can provide jobs. So you create these hot spots of instability. So even if all you care about is national security, these health things are a very cheap way to make sure you are not going to have turmoil that would eventually affect the whole world.

Gupta: Is there a diplomatic part of this? The fact that your foundation, others organizations and partnerships are doing this. Is that part of it?

Gates: The general idea of the rich helping the poor, I think, is important. That your sense of justice says, why should rich kids -- who barely get these diseases and almost never die of them -- why should they get the vaccines, when poor kids, who actually do die from these diseases, don't get those things? It's an unbelievable inequity that there isn't that access. It's been 15 years, usually, between when rich kids get vaccines and poor kids do.

Gupta: There was an article about concerns of corruption and fraud with regard to the Global Fund. Do you expect just to have a certain amount of corruption and fraud -- just say you know what, to do the work that we do, we have to expect and accept a certain amount of that?

Gates: Well the Global Fund does a fantastic job. [It has seen about 3% or 4% of the money it spends not be applied properly.] And they had a few grantees where the percentage was high enough that they cut them off and switched away from the government to another form of delivery. Because you don't want patients to die. You just have to find someone that does the honest delivery. So yes, you are going to have some. It's fine.

This is saving lives for well less than 1% of what you would spend in the rich world. And if you think lives are created equal -- this at least says well, are they at least worth 1%. And that's ignoring the sickness you avoid.

There was a survey recently that showed half the kids in Africa, because of infectious disease, have IQs of 80 or lower. That's cerebral malaria, that's malnutrition because their brain doesn't fully develop. And if you want them to be stable and on their own, you have got to make sure that terrible sickness, that permanently hurts them their entire life, is not there.

By and large, it is the one health intervention that can get to everyone. In fact, it is so simple, people often forget what a big deal this is. The 2 million people that would have died from smallpox now don't think, "Wow, I'm alive today because of vaccinations," but that's the case.