What is a coronavirus? - Elizabeth Cox

For almost a decade, scientists chased
the source of a deadly new virus

through China’s tallest mountains
and most isolated caverns.

They finally found it here: in the bats
of Shitou Cave.

The virus in question was a coronavirus

that caused an epidemic of severe acute
respiratory syndrome, or SARS, in 2003.

Coronaviruses are a group of viruses

covered in little protein spikes that
look like a crown—or corona in Latin.

There are hundreds of
known coronaviruses.

Seven of them infect humans, and can
cause disease.

The coronavirus SARS-CoV causes SARS,
MERS-CoV causes MERS,

and SARS-CoV-2 causes the
disease COVID-19.

Of the seven human coronaviruses,
four cause colds,

mild, highly contagious infections
of the nose and throat.

Two infect the lungs, and cause much
more severe illnesses.

The seventh, which causes COVID-19,
has features of each:

it spreads easily, but can severely
impact the lungs.

When an infected person coughs, droplets
containing the virus spray out.

The virus can infect a new person when
the droplets enter their nose or mouth.

Coronaviruses transmit best in
enclosed spaces,

where people are close together.

Cold weather keeps their delicate casing
from drying out,

enabling the virus to survive for longer
between hosts,

while UV exposure from sunlight
may damage it.

These seasonal variations matter
more for established viruses.

But because no one is yet immune
to a new virus,

it has so many potential hosts that it
doesn’t need ideal conditions to spread.

In the body, the protein spikes embed in
the host’s cells and fuse with them—

enabling the virus to hijack the host
cell’s machinery

to replicate its own genes.

Coronaviruses store their genes on RNA.

All viruses are either RNA
viruses or DNA viruses.

RNA viruses tend to be smaller,
with fewer genes,

meaning they infect many hosts and
replicate quickly in those hosts.

In general, RNA viruses don’t have
a proofreading mechanism,

whereas DNA viruses do.

So when an RNA virus replicates,

it’s much more likely to have
mistakes called mutations.

Many of these mutations are useless or
even harmful.

But some make the virus better suited
for certain environments—

like a new host species.

Epidemics often occur when a virus
jumps from animals to humans.

This is true of the RNA viruses that
caused the Ebola, zika,

and SARS epidemics and the
COVID-19 pandemic.

Once in humans, the virus still mutates—

usually not enough to create a new virus,

but enough to create variations,
or strains, of the original one.

Coronaviruses have a few key differences
from most RNA viruses.

They’re some of the largest, meaning
they have the most genes.

That creates more opportunity for
harmful mutations.

To counteract this risk, coronaviruses
have a unique feature:

an enzyme that checks for replication
errors and corrects mistakes.

This makes coronaviruses much more
stable,

with a slower mutation rate,
than other RNA viruses.

While this may sound formidable,

the slow mutation rate is actually a
promising sign

when it comes to disarming them.

After an infection, our immune systems
can recognize germs

and destroy them more quickly if they
infect us again

so they don’t make us sick.

But mutations can make a virus less
recognizable to our immune systems—

and therefore more difficult to fight off.

They can also make antiviral drugs
and vaccines less effective,

because they’re tailored very
specifically to a virus.

That’s why we need a new flu vaccine
every year—

the influenza virus mutates so quickly
new strains pop up constantly.

The slower mutation rate of coronaviruses
means our immune systems,

drugs, and vaccines might be able to
recognize them for longer after infection,

and therefore protect us better.

Still, we don’t know how long our bodies
remain immune to different coronaviruses.

There’s never been an approved treatment
or vaccine for a coronavirus.

We haven’t focused on treating the ones
that cause colds,

and though scientists began developing
treatments for SARS and MERS,

the epidemics ended before those
treatments completed clinical trials.

As we continue to encroach on other
animals’ habitats,

some scientists say a new coronavirus
jumping to humans is inevitable—

but if we investigate these unknowns,
it doesn’t have to be devastating.