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

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through China’s tallest mountains 
and most isolated caverns.

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

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The virus in question was a coronavirus

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that caused an epidemic 
of severe acute respiratory syndrome,

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or SARS, in 2003.

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Coronaviruses are a group of viruses

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covered in little protein spikes 
that look like a crown—

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or "corona" in Latin.

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There are hundreds 
of known coronaviruses.

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Seven of them infect humans, 
and can cause disease.

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

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

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Of the seven human coronaviruses, 
four cause colds,

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mild, highly contagious infections 
of the nose and throat.

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

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

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it spreads easily, 
but can severely impact the lungs.

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

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

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Coronaviruses transmit best 
in enclosed spaces,

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where people are close together.

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Cold weather keeps their delicate casing
from drying out,

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enabling the virus to survive 
for longer between hosts,

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while UV exposure from sunlight 
may damage it.

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These seasonal variations matter more 
for established viruses.

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

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

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

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enabling the virus to hijack 
the host cell’s machinery

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to replicate its own genes.

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Coronaviruses store their genes on RNA.

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All viruses are either RNA viruses 
or DNA viruses.

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RNA viruses tend to be smaller, 
with fewer genes,

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

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

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whereas DNA viruses do.

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So when an RNA virus replicates,

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

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Many of these mutations are useless 
or even harmful.

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But some make the virus better suited
for certain environments—

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like a new host species.

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

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This is true of the RNA viruses 
that caused

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the Ebola, Zika, and SARS epidemics, 
and the COVID-19 pandemic.

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Once in humans, the virus still mutates—

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usually not enough to create a new virus,

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

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Coronaviruses have a few key differences 
from most RNA viruses.

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

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That creates more opportunity 
for harmful mutations.

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To counteract this risk, 
coronaviruses have a unique feature:

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an enzyme that checks for replication 
errors and corrects mistakes.

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This makes coronaviruses 
much more stable,

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with a slower mutation rate, 
than other RNA viruses.

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While this may sound formidable,

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the slow mutation rate 
is actually a promising sign

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when it comes to disarming them.

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After an infection, our immune systems 
can recognize germs

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and destroy them more quickly 
if they infect us again

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so they don’t make us sick.

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

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and therefore more difficult to fight off.

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They can also make antiviral drugs 
and vaccines less effective,

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because they’re tailored 
very specifically to a virus.

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

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

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The slower mutation rate 
of coronaviruses means

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our immune systems, drugs, 
and vaccines

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might be able to recognize them 
for longer after infection,

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and therefore protect us better.

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

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

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

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and though scientists began developing
treatments for SARS and MERS,

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the epidemics ended before those 
treatments completed clinical trials.

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As we continue to encroach 
on other animals’ habitats,

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

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