When you hear medical experts talk about what most concerns them, you might be surprised to hear the word "flu."

The world may be due for an outbreak of pandemic influenza, a lethal virus that can sweep quickly across the globe. "And when it comes, it will affect every human alive today," writes CNN's Dr. Sanjay Gupta.

For years, Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, has called the pandemic influenza virus his biggest concern.

"If the virus acquires the ability to transmit readily among humans, an influenza pandemic could ensue, with the potential to kill millions of people," Fauci writes in the journal Emerging Infectious Diseases. "Reports in both the popular press and scientific literature have raised alarms in the United States and throughout the world. The prospect of pandemic influenza provides good reason to be concerned."

Most people are familiar with seasonal flu, a virus that mutates a little each year. There's an annual vaccine that has varying levels of success and although the flu can be dangerous to some demographics, the impact is typically not incredibly severe because our immune systems have seen some iteration of the virus and are equipped to combat it, Gupta says.

But the pandemic flu is totally different. It's a worldwide outbreak of a new influenza virus that's very different from recently circulating viruses. It spreads easily and quickly from person to person, according to the Centers for Disease Control and Prevention. The name "pandemic" translates to "all people."

The pandemic flu in history

Just in the 20th century, we've had four serious flu pandemics. The Spanish Flu of 1918 spread through North America, Europe and Asia. It's estimated that about one-third of the world became ill and between 50-100 million people died. In the U.S. alone, more than 675,000 people died in just eight months.

Four decades later, the Asian flu of 1958-59 had an estimated death toll of 1.1 million worldwide and 116,000 in the U.S.

In the 1968 pandemic, the estimated number of deaths was 1 million worldwide and about 100,000 in the U.S.

Then in 2009, it was the H1N1 pandemic. The CDC estimated that between 151,700 and 575,400 people worldwide died during the first year the virus circulated.

Where do pandemic flu viruses come from?

At times, various animals — including birds and pigs — have been hosts to pandemic influenza viruses. These viruses traditionally don't affect people but have mutated and spread to humans. The pandemic flu can also start as an already circulating virus that has mutated.

Are there vaccines to protect against the pandemic flu?

It's unlikely that the seasonal flu vaccine would protect against the pandemic flu virus, says the CDC. The federal government has stockpiled a few vaccines against some influenza viruses that have "pandemic potential." But it's likely that a new vaccine would need to be developed, and that would probably take at least six months. People would need two doses of the new vaccine to be protected.

"I have become convinced that if we can develop and deploy a pandemic flu vaccine just 24 weeks faster than is currently projected, the impact could change the course of human history," Gupta says. "Twenty-four weeks faster could mean the difference between 20,000 people dying in the next flu pandemic or more than 20 million people dying."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.