(CNN) Experts say we are "due" for one. When it happens, they tell us, it will probably have a greater impact on humanity than anything else currently happening in the world.
And yet, like with most people, it is probably something you haven't spent much time thinking about. After all, it is human nature to avoid being consumed by hypotheticals until they are staring us squarely in the face.
Such is the case with a highly lethal flu pandemic. And when it comes, it will affect every human alive today.
Pandemic flu is apolitical and does not discriminate between rich and poor. Geographical boundaries are meaningless, and it can circle the globe within hours. In terms of potential impact on mankind, the only thing that comes close is climate change. And, like climate change, pandemic flu is so vast, it can be challenging to wrap your head around it.
When most people hear "flu," they typically think of seasonal flu. No doubt, seasonal flu can be deadly, especially for the very young and old, as well as those with compromised immune systems.
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For most people, however, the seasonal flu virus, which mutates just a little bit every year, is not particularly severe because our immune systems have already probably seen a similar flu virus and thus know how to fight it. It's called native immunity or protection, and almost all of us have some degree of it.
Babies are more vulnerable because they haven't been exposed to the seasonal flu and older people because their immune systems may not be functioning as well.
Pandemic flu is a different animal, and you should understand the difference.
Panˈdemik/: pan means "all"; demic (or demographic) means "people." It is well-named, because pandemic flu spreads easily throughout the world. Unlike seasonal flu, pandemics occur when a completely new or novel virus emerges. This sort of virus can emerge directly from animal reservoirs or be the result of a dramatic series of mutations -- so-called reassortment events -- in previously circulating viruses.
In either case, the result is something mankind has never seen before: a pathogen that can spread easily from person to defenseless person, our immune systems never primed to launch any sort of defense.
Our only hope
With pandemic flu, we cannot solely rely on our bodies' ability to fight. A vaccine is our only real hope.
That fact is part of the reason the number 24 has been stuck in my head the past few months as I have been working on the CNN original film "Unseen Enemy." 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.
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.
Flu spreads across 40 states
In the 20th century alone, we experienced serious flu pandemics. The Spanish Flu of 1918 was an unprecedented catastrophe. Experts' best estimates (before there was more formal census and record-keeping) were that 20% to 40% of the world became ill and more than 50 million people died.
In the United States alone, approximately 675,000 people died in just the eight months between September 1918 and April 1919. Some people died the same day they became ill, and others died within a few days from complications of the flu, such as pneumonia or meningitis.
Forty years later, the Asian flu of 1958 and 1959 had a global death toll as high as 2 million and an estimated 70,000 of those in the US alone.
Early in 1968, the year before I was born, the Hong Kong flu began. By September, it made its way around the world, including the United States, and became widespread by December. It is believed that the number of those infected peaked during the fall, when kids were at school, transmitting the virus more freely.
Still, as many as a million people died, 34,000 in the US alone between September 1968 and March 1969.
We are overdue
We have learned a lot over the past 100 years and are better prepared because of it. In the developed world, at least, we have the ability to ease suffering with antivirals, breathing machines and antibiotics for secondary bacterial infections.
Experts can also more quickly characterize pathogens causing outbreaks. Although it took several years to identify the virus causing AIDS, for example, it took only a few weeks to identify the SARS virus.
We are also much faster at developing vaccines. According to research compiled by the science publication Mosaic, a typhoid vaccine was first tested in 1896 but only developed into a safe and effective vaccine nearly a hundred years later, in 1994. It took just 12 years, however, to develop a safe and effective Ebola vaccine (2003 to 2015). And, last week, the National Institute of Allergy and Infectious Diseases announced initiation of a phase II trial for a Zika vaccine that started in August with the hope it may be ready for emergency use by 2018.
Most important, our production capabilities have increased tremendously around the world. In 2006, according to the World Health Organization, we had the ability to make only 350 million doses of flu vaccine, but now, in theory, we could make 5.4 billion doses of flu vaccine if we harnessed and aligned manufacturing facilities all over the world.
24 weeks faster?
Would it ever be possible, however, to create and distribute a vaccine 24 weeks faster -- in six weeks instead of 30 weeks -- and save more than 20 million lives?
For starters, the way we typically make flu vaccine is still pretty antiquated and hasn't changed much in nearly 70 years.
We rely largely on hens' eggs to incubate and replicate the virus, which is too slow of a process to respond rapidly to pandemic flu.
Are we ready for the next global epidemic?
Richard Hatchett, CEO of the Coalition for Epidemic Preparedness Innovations, told me that recent scientific advances, however, are fully transforming the speed at which vaccines can be developed and approved.
Genomic techniques can be used to map the DNA or RNA of a new pathogen, genetically to engineer and mass-produce the same DNA or RNA and then inject it into the human body, leading to the production of antibodies to fight the virus. This method results in a new type of vaccine, a DNA vaccine.
Faster development and production is one critical step to shortening the vaccine process by 24 weeks.
Another is more robust surveillance systems everywhere in the world. As we were reminded with Ebola and Zika, an infection anywhere can be an infection everywhere.
The first time an ill patient shows up at a hospital anywhere on the planet with a novel virus, the clock starts ticking.
Improved surveillance means that hospital staff has to be astute enough to recognize that the patient doesn't fit the normal patterns we were taught in medical school and start to investigate further.
After that, there has to be speedy and accurate identification of the virus, purification and sequencing of the genetic material and then immediate sharing of the knowledge (via the cloud) enabling researchers everywhere to get to work on the development of new vaccines. After this genetically engineered vaccine is developed, it would have to be manufactured in the billions and then distributed to every city in the world.
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To make this a reality, it will take unprecedented partnerships between governments, even those at odds with each other. It will take the public and private sector working together every step of the way to ensure that the newly created vaccine can be distributed, trusted and administered.
It will take research and development money, as it always does, to bolster the techniques and infrastructure I described and make them available everywhere.
And it will take accepting that a pandemic of flu is likely in our lifetime. But with preparation and imagination, it need not be as nearly catastrophic as it was 50 or 100 years ago.
Make no mistake, outbreaks -- sudden, unexpected and localized eruptions of infection -- are inevitable. But, as epidemiologist Larry Brilliant (who spearheaded the effort to eradicate Small Pox), puts it: pandemics can be optional.
This is how.