0:00:07.828,0:00:12.526 For almost a decade, scientists chased [br]the source of a deadly new virus 0:00:12.526,0:00:16.526 through China’s tallest mountains [br]and most isolated caverns. 0:00:16.526,0:00:21.256 They finally found it here: in the bats [br]of Shitou Cave. 0:00:21.256,0:00:23.486 The virus in question was a coronavirus 0:00:23.486,0:00:30.739 that caused an epidemic of severe acute[br]respiratory syndrome, or SARS, in 2003. 0:00:30.739,0:00:32.889 Coronaviruses are a group of viruses 0:00:32.889,0:00:38.233 covered in little protein spikes that [br]look like a crown—or corona in Latin. 0:00:38.233,0:00:41.213 There are hundreds of [br]known coronaviruses. 0:00:41.213,0:00:45.320 Seven of them infect humans, and can [br]cause disease. 0:00:45.320,0:00:51.866 The coronavirus SARS-CoV causes SARS, [br]MERS-CoV causes MERS, 0:00:51.866,0:00:56.859 and SARS-CoV-2 causes the [br]disease COVID-19. 0:00:56.859,0:01:01.229 Of the seven human coronaviruses, [br]four cause colds, 0:01:01.229,0:01:05.323 mild, highly contagious infections [br]of the nose and throat. 0:01:05.323,0:01:10.081 Two infect the lungs, and cause much [br]more severe illnesses. 0:01:10.081,0:01:14.571 The seventh, which causes COVID-19, [br]has features of each: 0:01:14.571,0:01:18.929 it spreads easily, but can severely [br]impact the lungs. 0:01:18.929,0:01:23.661 When an infected person coughs, droplets[br]containing the virus spray out. 0:01:23.661,0:01:28.496 The virus can infect a new person when[br]the droplets enter their nose or mouth. 0:01:28.496,0:01:31.786 Coronaviruses transmit best in [br]enclosed spaces, 0:01:31.786,0:01:33.456 where people are close together. 0:01:33.456,0:01:36.866 Cold weather keeps their delicate casing[br]from drying out, 0:01:36.866,0:01:40.156 enabling the virus to survive for longer [br]between hosts, 0:01:40.156,0:01:43.744 while UV exposure from sunlight [br]may damage it. 9:59:59.000,9:59:59.000 These seasonal variations matter [br]more for established viruses. 9:59:59.000,9:59:59.000 But because no one is yet immune [br]to a new virus, 9:59:59.000,9:59:59.000 it has so many potential hosts that it[br]doesn’t need ideal conditions to spread. 9:59:59.000,9:59:59.000 In the body, the protein spikes embed in [br]the host’s cells and fuse with them— 9:59:59.000,9:59:59.000 enabling the virus to hijack the host[br]cell’s machinery 9:59:59.000,9:59:59.000 to replicate its own genes. 9:59:59.000,9:59:59.000 Coronaviruses store their genes on RNA. 9:59:59.000,9:59:59.000 All viruses are either RNA [br]viruses or DNA viruses. 9:59:59.000,9:59:59.000 RNA viruses tend to be smaller, [br]with fewer genes, 9:59:59.000,9:59:59.000 meaning they infect many hosts and [br]replicate quickly in those hosts. 9:59:59.000,9:59:59.000 In general, RNA viruses don’t have [br]a proofreading mechanism, 9:59:59.000,9:59:59.000 whereas DNA viruses do. 9:59:59.000,9:59:59.000 So when an RNA virus replicates, 9:59:59.000,9:59:59.000 it’s much more likely to have [br]mistakes called mutations. 9:59:59.000,9:59:59.000 Many of these mutations are useless or [br]even harmful. 9:59:59.000,9:59:59.000 But some make the virus better suited[br]for certain environments— 9:59:59.000,9:59:59.000 like a new host species. 9:59:59.000,9:59:59.000 Epidemics often occur when a virus [br]jumps from animals to humans. 9:59:59.000,9:59:59.000 This is true of the RNA viruses that [br]caused the Ebola, zika, 9:59:59.000,9:59:59.000 and SARS epidemics and the [br]COVID-19 pandemic. 9:59:59.000,9:59:59.000 Once in humans, the virus still mutates— 9:59:59.000,9:59:59.000 usually not enough to create a new virus, 9:59:59.000,9:59:59.000 but enough to create variations, [br]or strains, of the original one. 9:59:59.000,9:59:59.000 Coronaviruses have a few key differences [br]from most RNA viruses. 9:59:59.000,9:59:59.000 They’re some of the largest, meaning [br]they have the most genes. 9:59:59.000,9:59:59.000 That creates more opportunity for [br]harmful mutations. 9:59:59.000,9:59:59.000 To counteract this risk, coronaviruses [br]have a unique feature: 9:59:59.000,9:59:59.000 an enzyme that checks for replication [br]errors and corrects mistakes. 9:59:59.000,9:59:59.000 This makes coronaviruses much more [br]stable, 9:59:59.000,9:59:59.000 with a slower mutation rate, [br]than other RNA viruses. 9:59:59.000,9:59:59.000 While this may sound formidable, 9:59:59.000,9:59:59.000 the slow mutation rate is actually a [br]promising sign 9:59:59.000,9:59:59.000 when it comes to disarming them. 9:59:59.000,9:59:59.000 After an infection, our immune systems [br]can recognize germs 9:59:59.000,9:59:59.000 and destroy them more quickly if they[br]infect us again 9:59:59.000,9:59:59.000 so they don’t make us sick. 9:59:59.000,9:59:59.000 But mutations can make a virus less [br]recognizable to our immune systems— 9:59:59.000,9:59:59.000 and therefore more difficult to fight off. 9:59:59.000,9:59:59.000 They can also make antiviral drugs [br]and vaccines less effective, 9:59:59.000,9:59:59.000 because they’re tailored very [br]specifically to a virus. 9:59:59.000,9:59:59.000 That’s why we need a new flu vaccine[br]every year— 9:59:59.000,9:59:59.000 the influenza virus mutates so quickly [br]new strains pop up constantly. 9:59:59.000,9:59:59.000 The slower mutation rate of coronaviruses[br]means our immune systems, 9:59:59.000,9:59:59.000 drugs, and vaccines might be able to [br]recognize them for longer after infection, 9:59:59.000,9:59:59.000 and therefore protect us better. 9:59:59.000,9:59:59.000 Still, we don’t know how long our bodies[br]remain immune to different coronaviruses. 9:59:59.000,9:59:59.000 There’s never been an approved treatment[br]or vaccine for a coronavirus. 9:59:59.000,9:59:59.000 We haven’t focused on treating the ones[br]that cause colds, 9:59:59.000,9:59:59.000 and though scientists began developing[br]treatments for SARS and MERS, 9:59:59.000,9:59:59.000 the epidemics ended before those [br]treatments completed clinical trials. 9:59:59.000,9:59:59.000 As we continue to encroach on other [br]animals’ habitats, 9:59:59.000,9:59:59.000 some scientists say a new coronavirus [br]jumping to humans is inevitable— 9:59:59.000,9:59:59.000 but if we investigate these unknowns, [br]it doesn’t have to be devastating.