NARRATOR: Viruses have lived on Earth for billions of years, mutating and adapting.

They've been around longer than animals, longer than plants.

They're neither dead, nor alive.

They're active inside a living thing, powerless out in the open, but rise again in another host.

Their goal is simple-- to persist.

They infect our cells, then replicate and spread.

Smallpox, AIDS, polio, these are just some of the diseases caused by viruses that have ravaged humankind.

Now we're facing a new disease, COVID-19, caused by the coronavirus.

Our bodies and communities are struggling to fight back.

[man sneezes] An individual virus particle, or virion, is invisible to our naked eye.

It takes roughly 1,000 coronavirus particles to span the width of a human hair.

It travels lightly.

A virus is just genetic material wrapped in a layer of protein and fat, spreading through the air in moisture droplets or on surfaces, finding its way into our eyes, noses, and mouths.

Inside, the coronavirus hijacks the cells in the back of our nose, replicating and spreading downward, infecting healthy cells along the way.

Some viruses, like the ones that cause the common cold, infect our nose and throat.

Others can cause viral pneumonia.

That usually infects smaller areas of just one lung.

The coronavirus packs a vicious double punch.

It can infect the entire respiratory system all the way down to millions of tiny air sacs called alveoli.

Here is a single healthy alveolus.

Right next to it, there's a thin blood vessel, a capillary.

This is where one of our most life-sustaining exchanges happens.

The alveolus brings oxygen into the bloodstream and excretes carbon dioxide.

But the virus disrupts this whole process.

This immune cell, a macrophage, attacks it.

Sometimes it defeats the virus.

If our body needs more help, it recruits more immune cells, like these neutrophils.

While they're attacking the virus, they can end up injuring the alveolus too-- breaking down its walls.

Fluid rushes from the blood vessels into the alveolus, filling it up and blocking the exchange of oxygen.

Now it's much harder to breathe.

It's ultimately this two-pronged attack that makes the coronavirus so deadly-- the attack from the virus and our immune system's explosive response.

All this can lead to acute respiratory distress syndrome, what most people with COVID-19 die from.

Viruses don't benefit from wiping out their hosts.

They rely on us so they can exist.

To ultimately beat this coronavirus, we'll need an antiviral medication or immunity through a vaccine.

The impact from this pandemic has been devastating, but we can learn from it so we can stay a step ahead of the next one, because there are almost certainly more to come.

[slow music]