With the United Nations warning that a bird flu pandemic could kill between 5 million and 150 million people worldwide, the Bush administration warned Tuesday that the United States was not immune from the threat.

At a press conference, the president said he was "very concerned about what an avian flu outbreak could mean for the United States," suggesting that military-enforced quarantines may be necessary should the disease arrive in the United States.

Health and Human Services Secretary Mike Leavitt told FOX News that the United States was not prepared to handle an outbreak.

"We're not. And no one in the world is," Leavitt said.

Asian nations have slaughtered millions of birds in an effort to contain the avian flu epidemic sweeping through the region's poultry stocks, but the virus continues to infect humans. It has killed 64 people in Asia (search) to date.

Roughly 140 million birds worldwide carry the virus. It moves from wild to domestic birds, which then can infect people with whom they come in contact. So far, the virus has not "jumped" species, meaning that humans have only become infected through contact with a bird carrying the virus. However, as the virus spreads, mutations to the virus can occur that will allow it to spread from person to person. Once the virus can be transmitted among humans, it can become unstoppable. Moreover, because the virus originated in birds, humans have no immunity to it.

"The avian flu, or what scientists call the H5N1 virus (search), is of grave concern right now because it's spreading around the world," Leavitt said. "They call it a 'shift' and when a virus shifts, it moves into a status where humans literally have no immunity. And so it creates a potential for a pandemic flu," Leavitt said.

There have been no cases of the H5N1 virus in the United States, but federal health officials remain alert. While there is no vaccine for bird flu, there is an anti-viral medicine called Tamiflu, and the government is scrambling to stockpile enough doses. Of the 20 million doses the government projects it will need, Leavitt said they currently have just two million. More doses have been ordered, but even should supplies be sufficient, distribution remains a problem.

"The process of getting them to a person within 48 hours of their symptoms being realized is a challenge," Leavitt said. "It's one we're working on," he said.

The president said Tuesday that he envisioned a scenario in which the military was called upon to enforce quarantines in areas where outbreaks occurred.

"If we had an outbreak somewhere in the United States, do we not then quarantine that part of the country, and how do you then enforce a quarantine?" the president asked. "Who best to be able to effect a quarantine? One option is the use of a military," he said.

With or without the military, however, some lawmakers fear that America's public health system is not up to the challenge of fighting this flu. Senate Democrats criticized the Bush administration Tuesday for not yet releasing a national plan instructing public health agencies on how to prepare for an outbreak.

In a letter sent to the president, 32 Senate Democrats warned that the nation was "dangerously unprepared" to respond to a bird flu outbreak because the "public health infrastructure is weak" and because advance planning has been "needlessly delayed."

HHS (search) Secretary Leavitt said such plans will be released in a few days.

Click on the video box at the top of this story to see a report by FOX News' Major Garrett.