The Deadliest Virus on Earth

In the 1970s thousands of Chickenheads rained 
from the sky in Europe, making foxes and other   wildlife confused and very happy. Why? They were 
filled with a vaccine to fight the deadliest   virus known to humanity – since the 1930s 
a rabies epidemic had been sweeping across   wildlife populations in Europe and humans wanted 
to finally get rid of the virus once and for all. Rabies is named after Lyssa, the 
ancient Greek spirit of mad rage,   and has been haunting us for at least 4000 
years. It can turn animals into angry beasts   and humans into zombies that fear water. But 
what makes Lyssa fascinating is not just how   bizarre and deadly its infection is, but also how 
incredibly good it is at avoiding our defenses.

Viruses exist on the edge between life and death, 
hardly more than a few genetic instructions that   need living cells to multiply. The lyssavirus is 
simple even for a virus: It has only five genes,   that is the instructions for five proteins that 
let it solve complex problems: Infect a mammal,   avoid its immune system, travel to its brain, 
make more of itself and infect new hosts. Let's see what happens if you get infected. It all starts with a bite, most likely by a 
dog carrying millions of viruses in its saliva,   pushing them deep into the tissue. The goal is 
your nerve cells, your neurons. They are living   electrochemical wires, transferring signals 
throughout your body, and can stretch for up   to 1.5 meters, with their cellular machinery 
on one end and a terminal on the other. The terminal is where cells talk to each other, 
by passing chemicals that convey information.   Lyssa probably binds to the receptors 
that are crucial for this process   and slips inside the unsuspecting nerve cells.

Inside, the virus has to solve a big problem. 
It needs to get to the cellular machinery to   take over the cell and make more viruses 
– and because neurons are pretty long,   this can be far away. There 
is a solution at hand though: Cells have microtubules spanning their 
insides that give them structural integrity.   But they also provide a track system 
for a specialized delivery system:   Dynein motors are actual motors that use energy 
and deliver packages. They are made from 50   different proteins, ten times more than the virus, 
and look like a little pair of shoes.

Lyssa uses   one of its five proteins to hijack this amazing 
system and order it to head for the nucleus. What is the immune system doing to prevent 
all of that? Well, unfortunately not much. Usually when a virus attacks your civilian cells 
are crucial in activating your immune response.   They notice that they have been infected and 
release hundreds of thousands of a special family   of proteins: The interferons that, well, interfere 
with viruses. We’ll have to simplify a lot,   but in a nutshell, Interferons alert your 
immune system to make antivirus weapons.   But they do much more: they tell civilian 
cells to turn down their protein factories   for a while – which means that viruses 
can’t replicate efficiently anymore.

And interferons tell your cells to become super 
transparent, which is important, because how   can your immune cells notice that your civilian 
cells are infected when viruses hide inside them? Your body solves this by creating display windows 
into their insides, called MHC class I molecules.   Cells constantly produce stuff to stay alive,   and to showcase to your immune cells what is 
going on inside them, they take random samples   of their products and put them into these 
tiny display windows to give a peek inside.

Interferon tells your cells to make WAY more 
display windows and become super transparent.   If a cell is infected and forced to make virus 
parts, your immune cells will see these parts   in a window and order the infected cell to 
kill itself – and all the viruses trapped   within. This is one of the most powerful 
methods of wiping out a viral infection. Unfortunately Lyssa blocks your neurons 
from making interferons and stays basically   invisible to your immune system. In contrast 
to many other viruses, when it replicates,   it doesn’t kill its host, which 
would also trigger alarm systems.   Instead it stealthily jumps from neuron to 
neuron, very slowly making its way to your brain. This phase can take weeks to 
months and very rarely even years   and depends on a bunch of things, like if the bite 
was in your face or foot or how many viruses got   into your muscles. Lyssa is a patient monster. 
Until it reaches its goal: Your brainstem. Finally, the immune system catches 
on that something isn’t right   and reacts. It dispatches some of your most 
powerful antivirus cells, Killer T Cells,   to seek and kill infected cells and wipe out the 

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