By Robyn Shelton,The Orlando Sentinel

ORLANDO, Fla. – Investigators from Washington to Bangkok are chasing a potential enemy they fear could kill millions worldwide in a matter of months.

The threat is called a pandemic – in which a new strain of influenza virus rapidly spreads across the globe, causing illness and death on a mass scale. Concerns about a pandemic have reached a peak in recent months as a bird virus known as H5N1 continues to thrive in Asia. So far, millions of chickens and waterfowl have been slaughtered to prevent its spread.

At least 117 people have fallen ill with the same infection, also called avian flu, and at least 60 of them have died. Scientists fear the virus could mutate into a form that easily passes from human to human, causing an outbreak as bad, or worse than, the Spanish flu pandemic that killed an estimated 50 million people in 1918.

As Secretary of Health and Human Services Michael Leavitt tours avian-flu hot spots in Asia this week, many Americans are pondering the dangers of a deadly global flu outbreak for the first time.

Those dangers are real. But some experts think they have been overblown.

Here are some questions to consider:

• What is pandemic flu? What is avian flu? Are they the same?

No, they are not. Pandemic is the general term used to describe a global outbreak of an influenza virus that has not struck people in the past. A pandemic is highly dangerous because the population has no natural immunity to the novel virus.

Avian flu is a specific influenza virus that infects birds. Flu viruses always have been common in birds. These days, when people talk about avian flu, they usually are referring to the bird influenza circulating mostly in Asia. More recently, the virus has spread to birds in Turkey and Romania. Beyond animals, health officials have documented more than 100 human cases of avian flu. Most of the sick have been in Vietnam and had direct contact with animals carrying the virus.

• Is the world in danger of a pandemic?

Yes, experts agree that a pandemic flu outbreak is inevitable. But no one can say when that will occur. It could happen now; it may not happen for decades.

Pandemics have struck at least 10 times in recorded history, with the most recent in 1968-69, when the Hong Kong flu caused about 750,000 deaths worldwide and 34,000 in the United States. The most deadly pandemic was the 1918 Spanish flu outbreak, estimated to have killed as many as 50 million people worldwide. Scientists say a new influenza virus can emerge at any time.

• What would happen if a pandemic occurred?

A pandemic has the potential to cause millions of deaths worldwide, huge economic losses and social upheaval.

“It is potentially the most colossal public-health event of our lifetimes,” said Kim Elliott, deputy director of Trust for America’s Health, which studies health issues and has been sounding the alarm on pandemic flu for years.

Depending on its severity, a pandemic could overwhelm hospitals, close schools and curtail public events out of fears of spreading the virus. Trust for America’s Health estimates that more than 540,000 Americans could die in the next pandemic.

• Will avian flu become pandemic?

Nobody can answer that with certainty. For now, the avian flu virus is not capable of causing a pandemic. The human cases almost exclusively have affected people in close contact with the blood and feces of infected chickens. In at least one case, however, it appears that the virus might have been passed from one person to another.

For avian flu to become a wide-scale danger, it would have to mutate in a way that allows it to be passed easily between people. That would require some daunting genetic gymnastics.