[Script Info]
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[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.83,0:00:12.53,Default,,0000,0000,0000,,For almost a decade, scientists chased \Nthe source of a deadly new virus
Dialogue: 0,0:00:12.53,0:00:16.53,Default,,0000,0000,0000,,through China’s tallest mountains \Nand most isolated caverns.
Dialogue: 0,0:00:16.53,0:00:21.26,Default,,0000,0000,0000,,They finally found it here: \Nin the bats of Shitou Cave.
Dialogue: 0,0:00:21.26,0:00:23.49,Default,,0000,0000,0000,,The virus in question was a coronavirus
Dialogue: 0,0:00:23.49,0:00:27.49,Default,,0000,0000,0000,,that caused an epidemic \Nof severe acute respiratory syndrome,
Dialogue: 0,0:00:27.49,0:00:30.74,Default,,0000,0000,0000,,or SARS, in 2003.
Dialogue: 0,0:00:30.74,0:00:32.89,Default,,0000,0000,0000,,Coronaviruses are a group of viruses
Dialogue: 0,0:00:32.89,0:00:36.43,Default,,0000,0000,0000,,covered in little protein spikes \Nthat look like a crown—
Dialogue: 0,0:00:36.43,0:00:38.23,Default,,0000,0000,0000,,or "corona" in Latin.
Dialogue: 0,0:00:38.23,0:00:41.21,Default,,0000,0000,0000,,There are hundreds \Nof known coronaviruses.
Dialogue: 0,0:00:41.21,0:00:45.32,Default,,0000,0000,0000,,Seven of them infect humans, \Nand can cause disease.
Dialogue: 0,0:00:45.32,0:00:51.87,Default,,0000,0000,0000,,The coronavirus SARS-CoV causes SARS, \NMERS-CoV causes MERS,
Dialogue: 0,0:00:51.87,0:00:56.86,Default,,0000,0000,0000,,and SARS-CoV-2 causes \Nthe disease COVID-19.
Dialogue: 0,0:00:56.86,0:01:01.23,Default,,0000,0000,0000,,Of the seven human coronaviruses, \Nfour cause colds,
Dialogue: 0,0:01:01.23,0:01:05.32,Default,,0000,0000,0000,,mild, highly contagious infections \Nof the nose and throat.
Dialogue: 0,0:01:05.32,0:01:10.08,Default,,0000,0000,0000,,Two infect the lungs, \Nand cause much more severe illnesses.
Dialogue: 0,0:01:10.08,0:01:14.57,Default,,0000,0000,0000,,The seventh, which causes COVID-19, \Nhas features of each:
Dialogue: 0,0:01:14.57,0:01:18.93,Default,,0000,0000,0000,,it spreads easily, \Nbut can severely impact the lungs.
Dialogue: 0,0:01:18.93,0:01:23.66,Default,,0000,0000,0000,,When an infected person coughs, \Ndroplets containing the virus spray out.
Dialogue: 0,0:01:23.66,0:01:28.50,Default,,0000,0000,0000,,The virus can infect a new person when\Nthe droplets enter their nose or mouth.
Dialogue: 0,0:01:28.50,0:01:31.79,Default,,0000,0000,0000,,Coronaviruses transmit best \Nin enclosed spaces,
Dialogue: 0,0:01:31.79,0:01:33.46,Default,,0000,0000,0000,,where people are close together.
Dialogue: 0,0:01:33.46,0:01:36.87,Default,,0000,0000,0000,,Cold weather keeps their delicate casing\Nfrom drying out,
Dialogue: 0,0:01:36.87,0:01:40.16,Default,,0000,0000,0000,,enabling the virus to survive \Nfor longer between hosts,
Dialogue: 0,0:01:40.16,0:01:43.74,Default,,0000,0000,0000,,while UV exposure from sunlight \Nmay damage it.
Dialogue: 0,0:01:43.74,0:01:47.48,Default,,0000,0000,0000,,These seasonal variations matter more \Nfor established viruses.
Dialogue: 0,0:01:47.48,0:01:50.39,Default,,0000,0000,0000,,But because no one is yet immune \Nto a new virus,
Dialogue: 0,0:01:50.39,0:01:55.39,Default,,0000,0000,0000,,it has so many potential hosts that it\Ndoesn’t need ideal conditions to spread.
Dialogue: 0,0:01:55.39,0:02:00.03,Default,,0000,0000,0000,,In the body, the protein spikes embed\Nin the host’s cells and fuse with them—
Dialogue: 0,0:02:00.03,0:02:02.90,Default,,0000,0000,0000,,enabling the virus to hijack \Nthe host cell’s machinery
Dialogue: 0,0:02:02.90,0:02:05.54,Default,,0000,0000,0000,,to replicate its own genes.
Dialogue: 0,0:02:05.54,0:02:08.62,Default,,0000,0000,0000,,Coronaviruses store their genes on RNA.
Dialogue: 0,0:02:08.62,0:02:12.68,Default,,0000,0000,0000,,All viruses are either RNA viruses \Nor DNA viruses.
Dialogue: 0,0:02:12.68,0:02:15.70,Default,,0000,0000,0000,,RNA viruses tend to be smaller, \Nwith fewer genes,
Dialogue: 0,0:02:15.70,0:02:20.26,Default,,0000,0000,0000,,meaning they infect many hosts \Nand replicate quickly in those hosts.
Dialogue: 0,0:02:20.26,0:02:24.46,Default,,0000,0000,0000,,In general, RNA viruses don’t have \Na proofreading mechanism,
Dialogue: 0,0:02:24.46,0:02:27.06,Default,,0000,0000,0000,,whereas DNA viruses do.
Dialogue: 0,0:02:27.06,0:02:29.36,Default,,0000,0000,0000,,So when an RNA virus replicates,
Dialogue: 0,0:02:29.36,0:02:33.40,Default,,0000,0000,0000,,it’s much more likely to have \Nmistakes called mutations.
Dialogue: 0,0:02:33.40,0:02:37.34,Default,,0000,0000,0000,,Many of these mutations are useless \Nor even harmful.
Dialogue: 0,0:02:37.34,0:02:40.47,Default,,0000,0000,0000,,But some make the virus better suited\Nfor certain environments—
Dialogue: 0,0:02:40.47,0:02:42.96,Default,,0000,0000,0000,,like a new host species.
Dialogue: 0,0:02:42.96,0:02:47.43,Default,,0000,0000,0000,,Epidemics often occur when a virus \Njumps from animals to humans.
Dialogue: 0,0:02:47.43,0:02:49.97,Default,,0000,0000,0000,,This is true of the RNA viruses \Nthat caused
Dialogue: 0,0:02:49.97,0:02:55.88,Default,,0000,0000,0000,,the Ebola, Zika, and SARS epidemics, \Nand the COVID-19 pandemic.
Dialogue: 0,0:02:55.88,0:02:58.74,Default,,0000,0000,0000,,Once in humans, the virus still mutates—
Dialogue: 0,0:02:58.74,0:03:00.100,Default,,0000,0000,0000,,usually not enough to create a new virus,
Dialogue: 0,0:03:00.100,0:03:05.32,Default,,0000,0000,0000,,but enough to create variations, \Nor strains, of the original one.
Dialogue: 0,0:03:05.32,0:03:09.98,Default,,0000,0000,0000,,Coronaviruses have a few key differences \Nfrom most RNA viruses.
Dialogue: 0,0:03:09.98,0:03:13.33,Default,,0000,0000,0000,,They’re some of the largest, \Nmeaning they have the most genes.
Dialogue: 0,0:03:13.33,0:03:16.93,Default,,0000,0000,0000,,That creates more opportunity \Nfor harmful mutations.
Dialogue: 0,0:03:16.93,0:03:21.07,Default,,0000,0000,0000,,To counteract this risk, \Ncoronaviruses have a unique feature:
Dialogue: 0,0:03:21.07,0:03:25.07,Default,,0000,0000,0000,,an enzyme that checks for replication \Nerrors and corrects mistakes.
Dialogue: 0,0:03:25.07,0:03:27.76,Default,,0000,0000,0000,,This makes coronaviruses \Nmuch more stable,
Dialogue: 0,0:03:27.76,0:03:31.37,Default,,0000,0000,0000,,with a slower mutation rate, \Nthan other RNA viruses.
Dialogue: 0,0:03:31.37,0:03:33.64,Default,,0000,0000,0000,,While this may sound formidable,
Dialogue: 0,0:03:33.64,0:03:36.62,Default,,0000,0000,0000,,the slow mutation rate \Nis actually a promising sign
Dialogue: 0,0:03:36.62,0:03:38.86,Default,,0000,0000,0000,,when it comes to disarming them.
Dialogue: 0,0:03:38.86,0:03:42.22,Default,,0000,0000,0000,,After an infection, our immune systems \Ncan recognize germs
Dialogue: 0,0:03:42.22,0:03:45.31,Default,,0000,0000,0000,,and destroy them more quickly \Nif they infect us again
Dialogue: 0,0:03:45.31,0:03:47.40,Default,,0000,0000,0000,,so they don’t make us sick.
Dialogue: 0,0:03:47.40,0:03:51.14,Default,,0000,0000,0000,,But mutations can make a virus \Nless recognizable to our immune systems—
Dialogue: 0,0:03:51.14,0:03:53.72,Default,,0000,0000,0000,,and therefore more difficult to fight off.
Dialogue: 0,0:03:53.72,0:03:57.62,Default,,0000,0000,0000,,They can also make antiviral drugs \Nand vaccines less effective,
Dialogue: 0,0:03:57.62,0:04:01.48,Default,,0000,0000,0000,,because they’re tailored \Nvery specifically to a virus.
Dialogue: 0,0:04:01.48,0:04:04.42,Default,,0000,0000,0000,,That’s why we need a new flu vaccine\Nevery year—
Dialogue: 0,0:04:04.42,0:04:09.25,Default,,0000,0000,0000,,the influenza virus mutates so quickly \Nthat new strains pop up constantly.
Dialogue: 0,0:04:09.25,0:04:11.94,Default,,0000,0000,0000,,The slower mutation rate \Nof coronaviruses means
Dialogue: 0,0:04:11.94,0:04:14.68,Default,,0000,0000,0000,,our immune systems, drugs, \Nand vaccines
Dialogue: 0,0:04:14.68,0:04:18.17,Default,,0000,0000,0000,,might be able to recognize them \Nfor longer after infection,
Dialogue: 0,0:04:18.17,0:04:21.03,Default,,0000,0000,0000,,and therefore protect us better.
Dialogue: 0,0:04:21.03,0:04:25.71,Default,,0000,0000,0000,,Still, we don’t know how long our bodies\Nremain immune to different coronaviruses.
Dialogue: 0,0:04:25.71,0:04:29.61,Default,,0000,0000,0000,,There’s never been an approved treatment\Nor vaccine for a coronavirus.
Dialogue: 0,0:04:29.61,0:04:32.33,Default,,0000,0000,0000,,We haven’t focused on treating \Nthe ones that cause colds,
Dialogue: 0,0:04:32.33,0:04:36.09,Default,,0000,0000,0000,,and though scientists began developing\Ntreatments for SARS and MERS,
Dialogue: 0,0:04:36.09,0:04:40.37,Default,,0000,0000,0000,,the epidemics ended before those \Ntreatments completed clinical trials.
Dialogue: 0,0:04:40.37,0:04:43.49,Default,,0000,0000,0000,,As we continue to encroach \Non other animals’ habitats,
Dialogue: 0,0:04:43.49,0:04:48.62,Default,,0000,0000,0000,,some scientists say a new coronavirus \Njumping to humans is inevitable—
Dialogue: 0,0:04:48.62,0:04:53.55,Default,,0000,0000,0000,,but if we investigate these unknowns, \Nit doesn’t have to be devastating.