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

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through China’s tallest mountains [br]and most isolated caverns.

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They finally found it here: [br]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 [br]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 [br]that look like a crown—

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or "corona" in Latin.

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There are hundreds [br]of known coronaviruses.

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Seven of them infect humans, [br]and can cause disease.

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The coronavirus SARS-CoV causes SARS, [br]MERS-CoV causes MERS,

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and SARS-CoV-2 causes [br]the disease COVID-19.

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Of the seven human coronaviruses, [br]four cause colds,

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mild, highly contagious infections [br]of the nose and throat.

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

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

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it spreads easily, [br]but can severely impact the lungs.

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

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

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Coronaviruses transmit best [br]in enclosed spaces,

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where people are close together.

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Cold weather keeps their delicate casing[br]from drying out,

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enabling the virus to survive [br]for longer between hosts,

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while UV exposure from sunlight [br]may damage it.

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These seasonal variations matter more [br]for established viruses.

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

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

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

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enabling the virus to hijack [br]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 [br]or DNA viruses.

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RNA viruses tend to be smaller, [br]with fewer genes,

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meaning they infect many hosts [br]and replicate quickly in those hosts.

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In general, RNA viruses don’t have [br]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 [br]mistakes called mutations.

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Many of these mutations are useless [br]or even harmful.

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But some make the virus better suited[br]for certain environments—

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like a new host species.

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

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This is true of the RNA viruses [br]that caused

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the Ebola, Zika, and SARS epidemics, [br]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, [br]or strains, of the original one.

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Coronaviruses have a few key differences [br]from most RNA viruses.

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

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That creates more opportunity [br]for harmful mutations.

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To counteract this risk, [br]coronaviruses have a unique feature:

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an enzyme that checks for replication [br]errors and corrects mistakes.

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This makes coronaviruses [br]much more stable,

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with a slower mutation rate, [br]than other RNA viruses.

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While this may sound formidable,

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the slow mutation rate [br]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 [br]can recognize germs

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and destroy them more quickly [br]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 [br]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 [br]and vaccines less effective,

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because they’re tailored [br]very specifically to a virus.

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

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

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The slower mutation rate [br]of coronaviruses means

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our immune systems, drugs, [br]and vaccines

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might be able to recognize them [br]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[br]remain immune to different coronaviruses.

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

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

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and though scientists began developing[br]treatments for SARS and MERS,

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the epidemics ended before those [br]treatments completed clinical trials.

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As we continue to encroach [br]on other animals’ habitats,

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

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