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A war has been going on for billions of years  that breeds well armed monsters, who struggle with  

other monsters for survival. Having no particular  interest in us, most of them are relatively  

harmless, as our immune systems deal with  their weapons easily. But there are exceptions.

Naegleria fowleri is an amoeba  that has not only developed a  

deadly taste for human brains but is also a  match for our defences and stars in dramatic  

headlines. What happens when  this monster enters your body?

Naegleria fowleri is an amoeba, a microbe with a  nucleus, one of the smallest lifeforms on earth.  

It is a voracious hunter of bacteria and other  critters that it devours whole and rips into  

pieces. Like many amoebea it is able to transform  into different stages that help it survive,  

but most of the time Naegleria  fowleri is in its trophozoite stage,  

during which it looks like a squishy blob  with tiny arms and hunts, divides and thrives.

Its natural home is in fresh water: ponds,  rivers, lakes and hot springs. But unfortunately  

it also feels happy in pipes, swimming pools,  fountains or spas when they are not properly  

treated. The warmer the water, the more it thrives  and multiplies. So in the summer, when humans seek  

to cool off and enjoy themselves, the chances  are highest that both species will interact.

Because this makes it hard to avoid, millions of  people regularly have contact with the amoeba,  

especially in warmer climates, and many people  even seem to have antibodies against it.  

And this is mostly ok, you can even  swallow it without consequences.

Things turn bad when people dive or swim in water  contaminated with the amoeba and water splashes  

high up into their noses. In a single drop  of lake water there are millions of viruses,  

bacteria and amoebea and that isn’t really a big  deal. But Naegleria fowleri is different. Let  

us zoom into the nose of an unsuspecting victim  enjoying a great summer day and see what happens.

First of all, the amoeba doesn’t  really want to be inside your nose  

as it is not really looking for trouble,  it just wants to eat a few bacteria.  

Instead it is greeted by your natural defenses.  Unfortunately for humans, Naegleria fowleri  

happens to be exceptionally good at generally  flying under the radar of your immune system.  

For example, the inside of your nose is covered  by mucosa, a slime filled with chemicals that  

kill or stun possible invaders or alert immune  cells. But Naegleria fowleri is not particularly  

bothered by them and instead calmly checks out the  scenery, mildly annoyed about the whole ordeal.

Now, if you are unlucky, the tiny  critter stumbles over something  

that actually sparks its interest: Nerve cells.

Your nose is filled with a large  network of olfactory nerve cells  

that pick up molecules from the outside and  transmit their information to your olfactory bulb,  

the center of smell in your brain. To do  their job these cells talk to each other by  

releasing various messenger chemicals and  recognizing them via specific receptors.  

One of the most important of  these chemicals is acetylcholine.

Through sheer evolutionary bad  luck, Naegleria fowleri happens to  

have receptors that recognize acetylcholine.  

And it seems to attract them irresistibly, a  little like moths that are attracted by light.

So as your olfactory nerve cells do their job,  using plenty of acetylcholine to talk to the  

brain, Naegleria fowleri enters your tissue. It  seems to follow the chemical signals upstream.  

Neutrophils, crazy suicide warriors begin to  attack the amoebae. Individually they have no  

chance against them as the invaders are large and  pretty buff fighters, used to dealing with tough  

enemies. So the defenders swarm the intruders  and kill them either by vomiting chemicals  

that punch holes into them or by literally  ripping parts of them off and devouring them.  

But the Naegleria fowleri train is still on track  and while the Neutrophil attacks slow them down,  

they continue to follow the olfactory  nerves to their final destination:

Your brain.

This process can take between one and nine days  

and you’ll probably not notice  anything during that time.  

Until the amoebae reach the olfactory bulb, the  center of smell and entrance to your brain. Your  

brain cells are nothing more than helpless victims  and they all release that wonderful acetylcholine.

Naegleria fowleri initiates a massacre and  releases an onslaught of various attack molecules.  

Some of them are basically little bombs  that rip holes into your cells on contact  

so their pieces can be eagerly  consumed. But Naegleria fowleri  

is now multiplying - and it’s also becoming  really creepy. In a feeding frenzy it can  

develop up to a dozen suckers called food cups,  that look like giant eerie mouths. The amoebae  

engage your brain cells, suck them in and  rip large “bites” out of them while they  

are still alive. Now things escalate quickly  and the disease that will kill you sets in.

Alerted by the massacre, millions  of immune cells, Neutrophils,  

Eosinophils and microglias invade the  infected tissue. Which is a problem:  

your immune system is dangerous and not  exactly a careful fighter. It’s like  

burning down a forest to kill the wolves  inside it. A really bad idea in the brain.

They waste no time and attack the amoeba,  using all the weapons available to them,  

from chemicals to trying to eat them  alive. Neutrophils explode themselves  

to erect barriers spiked with deadly  chemicals. A fierce battle ensues.  

Naegleria fowleri can actually fight back,  itself attacking and killing many immune cells.

The immune system now throws everything it has at  the invader but in vain. The complement system,  

tiny protein bombs that can kill intruders on  their own, are easily disabled. Antibodies,  

usually one of your superweapons, are just  destroyed or swallowed. A high fever that  

usually slows enemies down does nothing,  as the amoeba actually thrives in the heat.  

All the while the amoebae continue to  multiply, fight and devour your brain cells.

A disastrous chain reaction is taking place.  

One major thing your immune cells do when  they fight is to cause inflammation. Which  

directs large amounts of fluid from your  bloodstream into the site of an infection.  

So as the battle rages on without a clear  winner, more and more fluid enters the brain.

At this point the human will feel  symptoms that quickly escalate.  

It all begins pretty vaguely, a headache, fever,  

nausea and vomiting. As the battle spreads  rapidly through the brain serious symptoms appear,  

from confusion, inability to concentrate  to fatigue, seizures and hallucinations.  

The brain swells up massively but can’t expand due  to the bones surrounding it. So it compresses and  

disables the brainstem that controls things like  breathing. Usually within a week the patient dies.

Up to 97% of patients infected by the  amoeba share this fate. In almost all cases,  

by the time an infection by Naegleria fowleri is  recognized the disastrous battle for the brain  

is already so far along that there is almost  nothing to be done. Not only do we currently  

not have effective treatments, there are also an  abundance of open questions about how an amoeba  

that usually enjoys its life in open water, is  able to overcome our immune system so effectively.

So how worried do you need to be about this  horrifying killer amoeba? Well, not very. While  

the Naegleria fowleri is clearly extremely deadly  and the infection truly horrible, there have only  

been a few hundred cases in the last few decades.  You are way more likely to drown in a pool than  

to get infected. Not only does the amoeba need  to be flushed high up your nose, it also needs  

to get a good grip and it also has to make its  way through the first lines of your defences.

Ultimately Naegleria fowleri is neither evil  nor a huge public health risk. But every year  

some unlucky people have to deal with it. We still  have so much to learn about it and until we find a  

way to treat it, Naegleria fowleri will continue  to be this vague and horrifying thing, hunting  

in puddles and lakes and sometimes pools. Usually  for bacteria. And very occasionally, for people.

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