where you juuuust don’t feel quite right.

But influenza proper is a handful of specific viruses that invade the respiratory lining,

evade the immune system, and mutate at a rate that even the world’s most brilliant doctors,

scientists, and health organizations struggle to keep up with.

So what is Influenza exactly,

and why does it always seem to be one step ahead of us?

Some of the confusion around the term ‘flu’ may stem back to the word’s roots: “Influenza”

is an Italian word that just means,  ‘influence.'

Because, back 500 years ago, people didn't know about germs then.

They thought if you got this disease, you were under the influence, perhaps of the stars

or something else.

My name is David Morens.

I'm a medical doctor and an epidemiologist, and a virologist, and a historian, at the

National Institutes of Health, and an officer in the United States Public Health Service,

and I'm here today to talk about one of my favorite subjects: influenza.

Influenza presents with such iconic symptoms

that many other diseases are classified by their similarity to it.

Tell your doctor you’re experiencing ‘flu-like symptoms,’ and they’ll know exactly what

you mean: coughing, fever, head and muscle aches, fatigue, and well, just general “malaise...”

…which kind of means just feeling really bad.

Anybody who's ever had flu knows what it is.

But while many illnesses are similar to influenza, there’s only one True Flu.

Well, four, actually... and trying to put a finger on its countless variations and mutations

is actually where influenza starts to get the better of us.

Depending on how you cut the cake, there are billions and trillions and zillions of different

types of influenza.

But to start, we categorize them into four categories: Influenza A, B, C, and D. The

ones we're most interested in because they affect humans every year are A and B. C also

infects humans, but it's pretty much a minor disease that affects children, and D is mostly

an animal disease that people generally don't get, or at least they don't get symptoms from it.

Influenza A, B, and C for the kiddos, start by entering your upper respiratory tract,

either when you breathe contaminated air, or touch your nose and mouth after coming

into contact with the virus.

The only cells it really infects are what we call respiratory epithelial cells, a one-cell

layer of cells that kind of protect the breathing tubes and the lungs, all the way down to the

bottom, where we have air sacs.

Those lining cells are the ones that get infected.

Once they reach your respiratory lining, the viruses attach to a specific kind of cell

receptor, called sialic acid.

If you could see them up close, they'd look like branches of trees.

They stick out and they go in all different directions.

They kind of capture things, and by binding to it, it can be dragged inside the cell,

where it causes the infection.

The influenza virus then makes its way through your respiratory passages, in rare cases,

reaching all the way to the bottom of the lungs, and leading to severe pneumonia.

All this suspicious activity would normally cause your body to launch an immune response.

And usually, it tries to.

But this is complicated by the fact that the virus and the immune cells coming to the rescue

are separated by that epithelial barrier.

That's one of the reasons it makes it hard to develop strong immunity to flu, and to

raise strong immunity by using a flu vaccine.

And as if that weren’t enough, even if you are able to mount a defense of some sort against

the virus, influenza has already invented several thousand more versions of itself,

before you can blink an eye.

Influenza viruses are not stable things.

They're always moving and changing, day-to-day, hour-to-hour, minute-to-minute.

Mutation is all they can do.

There is no state of their existence in human beings that is not constantly mutating.

This is because of a machinery in their RNA that causes consistent errors in gene replication.

But, unlike for humans, these ‘errors’ actually works to the virus’s advantage.

They're good errors that help them to survive.

Even in your own body, if you are infected with an influenza virus today, you're not

going to be infected by one virus.

You're going to be infected by let us say 1,000 or 10,000 viruses.

And it’s this rapid-fire mutation that makes whipping up a vaccine for this set of viruses

a pretty tall order.

Each year, researchers from the World Health Organization and around the world develop

one vaccine for the Northern Hemisphere, and one for the Southern.

These vaccines are released six months apart, to coincide with winter’s ‘flu season’

in each area.

Their recipe is slightly different each time, and can be tailored for specific types of

patients, like the elderly.

The current vaccines have four viruses: two influenza A, and two influenza B.

And it may or may not work.

Because in the meantime, the viruses circulating around the world may have mutated to the point

where your best guess vaccine is worthless.

So, the scientists have to guess now, what vaccine they're going to need

six months from now.

It's like looking into a crystal ball.

And in addition to this seasonal cycle, every three decades or so, the stakes ramp up, and

the disease is dramatically more hostile, causing a pandemic.

The years we have a pandemic, as many as 50% or 60% of all people might be infected

within a year.

That virus causing a pandemic can come at any time of year.

Eventually, we don't call it a pandemic anymore - we call it a seasonal virus.

But all over the world, research continues on influenza, and the hope of a universal

vaccine could be in sight.

A lot of scientists, including NIH and CDC, are working very hard on this.

Every year, all over the world, thousands of influenza viruses are being taken from

patients, and sequenced.

This gives an idea about what directions the evolution of this virus is going.

By learning how the body can successfully control influenza, we have a better idea how

we can mimic that with a vaccine.

But in the meantime…

Don't walk, run and get your flu shot, every year.