Soldiers and police ruthlessly shooting down waves of zombies that flood from infected streets trying to escape and infect more cities.

This is what happens when your body fights cancer⏤more exciting than any movie.

How does this battle for survival unfold?

1. The Elimination Phase

It all begins with a single corrupted cell.

It's no longer able to repair its genetic code, it can't kill itself anymore, and it's beginning to multiply rapidly.

At this point, things are not great, not terrible; this cell is not yet dangerous, but if nothing happens, it soon will be.

Over a few weeks, the corrupted cell keeps making copies of itself.

One cell turns into dozens, hundreds, thousands.

Because the original was broken, its copies are breaking and mutating even more.

They turn into different genetic lineages, clans that are working together and competing. 

Some mutate in a way that makes them weaker, others' mutations don't change anything, while a few become fitter and better at survival.

Together, they now form a tiny, tiny tumor; not cancer yet, but getting there.

The growing tumor needs a lot of resources. 

If the cells don't get food and oxygen, they'll die, and the problem just solves itself. 

Unfortunately, a few corrupted cells unlock a new mutation that saves them:

The ability to order the growth of new blood vessels.

And, so, your body provides the supply they need to survive.

But as the tumor continues to grow, it starts causing damage.

Neighboring healthy cells begin to starve and die, which attracts attention.

In a sense, this tiny tumor is like a rogue town.

Imagine a group of rebels in Brooklyn decide that they're no longer part of New York,

and start a new settlement called Tumor Town, which happens to occupy the same space.

The new city wants to grow, so it orders tons of steel beams, cement, and drywall.

New buildings follow no logic, are badly planned, ugly, and dangerously crooked. 

They're built right in the middle of streets, on top of playgrounds, and on existing infrastructure.

The old neighborhood is torn down or overbuilt to make room for new stuff.

Many of the former residents are trapped in the middle of it and begin to starve.

This goes on for a while until the smell of death finally attracts attention.

Building inspectors and police show up.

In your body, attracted by the stench of dead cells, your immune system is activated.

The first responder immune cells invade the tumor.

The macrophages and natural killer cells police forces that go right to work, killing and eating tumor cells.

They release chemical signals that let the whole immune system know that there is cancer to be eradicated.

Dendritic cells, the intelligence officers of your immune system, collect samples of dead tumor cells and begin activating your heavy weapons:

Helper and killer T-cells.

We explained these specialized super weapons in another video,

but all you really need to know is that they have a library listing every bad thing that can come into your body.

While each cancer is unique, there are genetic corruptions that they can't hide.

And your T-cells know what to look for; they are the deadliest cancer killers you have.

By the time they arrive, the tumor has grown to hundreds of thousands of cells, but this is about to change.

T-cells block the growth of new blood vessels, which starves thousands of tumor cells and puts an end to their growth.

Imagine the building inspectors switching off electricity and water,

and putting up roadblocks to Cancer Town so no more food or materials can be delivered.

With no way to hide from the carnage unfolding, the tumor collapses as hundreds of thousands of tumor cells are massacred.

Their carcasses are cleaned up and consumed by macrophages that then order healthy tissue to regenerate.

Your body has crushed the illegal Tumor Town without mercy.

You'll never know about this fight or how many times this has happened inside your body.

Except in this case, something didn't go as planned.

2. The Equilibrium Phase

Unfortunately, natural selection spoils your victory.

By doing its best to destroy the tumor, your immune system accidentally selects the fittest tumor cells.

Remember, the tumor consists of different lineages that keep growing and keep mutating.

Most of these are eradicated, but just a few are more resilient.

One cell survives; it comes from the fittest tumor lineage and was just a bit better at surviving the massacre than anyone else.

It decides to do it all over again, but better this time.

This tumor cell is much stronger than any of the thousands that were killed.

Maybe it is better at hiding or fighting back.

Maybe it grows faster or is better at stealing resources.

Maybe it can survive with much less oxygen.

And so it all begins again.

It's like the surviving rebels that started Tumor Town have learned their lessons.

Now they know the law better and how to break it, what permits help them, and how to avoid the police.

And, so, the surviving tumor cell makes thousands of copies that mutate and form new lineages,

until, once again, a tumor has grown, made up of more resilient cells.

The immune system doesn't care, though, and this time, it even has experience.

Instead of starting with police, swat teams go right in to tear Tumor Town down, killing its inhabitants without mercy.

But, once again, they don't get everyone; one of them survives⏤a fitter tumor cell from an already fitter lineage.

This time, it gets a cheap suit and studies the building code, pretending to be a lawyer to start Tumor Town all over again.

This struggle now repeats a few times, each time, the rebels learn a bit more about how to avoid the law.

If, at any point, the immune system gets all of the tumor cells, the story ends.

But in this case, it doesn't.

Finally, a tumor cell changes in a way that makes it properly dangerous⏤cancer.

The type that kills people.

How?

Immune cells have an off switch that deactivates them before they can attack, which, in principle, is a good idea.

The immune system is extremely dangerous, and, in many cases, it needs to be shut down, like around your central nervous system.

But this off switch can be exploited.

The mutated tumor cell finds a way to switch the immune system off by targeting inhibitor receptors on anti-cancer cells.

Inhibitor receptors stop immune cells from, well, killing. 

This cell is now the powerful founder of a new lineage of cancer cells,

and mass produces thousands of new copies that once again change and mutate further, building yet another Tumor Town.

3. The Escape Phase

The new cancer cells have become immune to the immune system, and everything is different this time.

Tumor Town has been rebuilt, even uglier and stranger than before, but now, the cancer city council has forged all sorts of permits.

As building inspectors come to shut down construction, they get confused.

Stunned, they wander off, unable to order the destruction of the sprawling buildings.

Police try to enter the illegal city to arrest the builders and execute inhabitants.

But this time, Tumor Town has erected its own roadblocks that keep the law from entering.

Confused officers stand around helplessly.

As Tumor Town slowly envelops the former Brooklyn and more and more civilians die, T-cell swat teams arrive to end this travesty.

But things get worse.

New lineages of Tumor Town officials have started to forge court documents that order police to shoot at the swat teams.

What the cancer cells are doing at this point is actively shutting down immune defenses by sending corrupt signals.

The now malignant tumor is no longer a pushover and has begun creating the cancer microenvironment⏤

a sort of borderland that's hard to cross.

All avenues of attack have been shut down, and uncontrollable growth is the consequence.

This is a dangerous tumor.

Cells that are strong and able to fight, push your immune system back, and expand further. 

If more mutations happen, then some of the cancer cells will begin to explore the world and expand into other tissues to build new towns.

And this is exactly what makes cancer so harmful:

It's taking up space and stealing so many nutrients that your true self has no room to function anymore.

If this goes on for too long, organs will shut down.

But this tactic is a dead end.

The more successful cancer gets, the more damage it does to its world.

When the body dies, the cancer dies, too; it truly is a game without winners.

Except, humanity is planning to win this game.

At this very moment, hundreds of thousands of scientists are working on new and better ways of killing cancer,

to destroy and burn down Tumor Towns for good.

In recent years, immunotherapy has made enormous progress.

It's a relatively new therapy in which your own immune cells are modified to kill cancer better than any medicine can do.

It's like giving your building inspectors machine guns and flamethrowers.

But this is a story for another time.

For now, cancer is a battleground.

But if human ingenuity is to be trusted, then, one day, maybe in the not-too-distant future, we will eradicate it once and for all.

This video was made possible in part by direct viewer support and in part through a grant by Gates Ventures.

Thanks a lot for their support!

Please check out our source document for more background and in-depth information.