WASHINGTON -- Since it was detected a decade ago, the H5-N1 bird flu virus has spread halfway around the world.

Now, experts think it could be just a matter of months before it arrives in the Western Hemisphere, threatening the nation’s poultry industry and potentially millions of people.

"If this virus does arrive, I think we need to emphasize the fact that this is still very much a bird disease, and a virus in birds does not signal that a pandemic has started," Dr. Ron DeHaven of the U.S. Dept. of Agriculture said.

While hundreds of people have gotten sick overseas, researchers said that the virus still isn't able to travel person to person efficiently.

The thing that health experts fear is a mutation of the virus that allows human-to-human transmission. Should that happen, global pandemic would likely follow, they say.

"The fear is that you'll have something like 1918, the Spanish flu pandemic," Jim Young, the president of vaccine maker MedImmune, said. "Tens of millions of people died. It wasn't the young or the elderly, it was healthy young adults that were dying from this."

"It could affect 90 million people in the U.S. getting sick," U.S. Department of Health and Human Services Mike Leavitt said. "About 45 million would require medical condition and 2 million could die."

The biggest question is "when?"

No one knows if a pandemic could start in two weeks or two years.

But most experts agree that a global outbreak will eventually hit and that now is the time to get ready.

Public health officials are focusing most on the H5-N1 strain of bird flu.

But they said that may not be the version of the virus that sparks a pandemic.