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Another appears to make the spike protein less fragile, and in lab experiments with cell cultures, it makes the virus more infectious. One is in a region called the polybasic cleavage site, which is known to make other coronaviruses and flu viruses more infectious. This suggests that the mutation first arose either in pangolins or an as yet unidentified species and happened to allow the virus to jump over to people, too.īut SARS-CoV-2 carries other changes in the spike protein that appear to have arisen after it jumped to people, since they don’t occur in the bat or pangolin viruses. Compared with the virus in bats, both the virus that infects people and a close relative in pangolins carry a mutation that changes the shape of the surface “ spike protein.” The alteration is right at the spot that binds to host cell receptors to let the virus in. SARS-CoV-2 shows these two stages clearly. That’s what elevates a virus from an occasional nuisance to one capable of causing widespread harm. But to become capable of causing epidemics, the virus also has to become infectious - that is, transmissible between individuals - in its new host. First, the virus has to be able to invade the new host’s cells: That’s a minimum requirement for making the host sick. Host switching actually involves two steps, though these can overlap. Three sites are shown because the spike protein is composed of three identical subunits that bind together. This added robustness appears to make the virus more infectious. Here are examples.Ī recent mutation alters the SARS-CoV-2 spike protein to make it less fragile (the altered bits are shown as colored blobs). Pandemics - disease outbreaks of global reach - have visited humanity many times. Any or all of these factors are likely to differ from one host species to another, so viruses will need to change genetically - that is, evolve - in order to set up shop in a new animal. Once inside the cell, the virus has to evade the cell’s immune defenses and then commandeer the appropriate parts of the host’s biochemistry to churn out new viruses. To get into a host cell, a molecule on the virus’s surface has to match a receptor on the outside of the cell, like a key fitting into a lock. So, too, for coronaviruses: The ones behind SARS (severe acute respiratory syndrome), MERS (Middle East respiratory syndrome) and Covid-19 all probably originated in bats and arrived in people via another, stepping-stone species, likely palm civets, camels and possibly pangolins, respectively.īut making the jump from one species to another isn’t easy, because successful viruses have to be tightly adapted to their hosts. Many of the scariest viruses that have caused past or current epidemics originated in other animals and then jumped to people: HIV from other primates, influenza from birds and pigs, and Ebola probably from bats. “It depends entirely on the biological nuance of the situation.” Steps to viral success “One thing you learn about evolution is never to generalize,” says Edward Holmes, an evolutionary virologist at the University of Sydney, Australia, and author of an article on the evolution of emerging viruses in the Annual Review of Ecology, Evolution, and Systematics. The outcome depends on the complex and sometimes subtle interplay of ecological and evolutionary forces that shape how viruses and their hosts respond to one another. But it could also remain a serious threat or perhaps even evolve to become more lethal. It might end up as just another cold virus, as may have happened to another coronavirus in the past. It’s possible the virus could lose its lethal character and settle into an evolutionary détente with humanity. Why did SARS go away while today’s coronavirus just keeps on spreading? Why, for that matter, did both these coronaviruses spill over into people at all, from their original bat hosts?Īnd just as vital as those questions is another: What happens next?Īs we face the current pandemic, it will be important to understand how SARS-CoV-2, the virus that causes Covid-19, is likely to evolve in the months and years ahead. No cases of SARS have been reported since 2004.Ĭontrast that with the closely related coronavirus that causes Covid-19 today: more than 13,600,000 confirmed cases as of July 16, and more than 585,000 deaths. By June, the outbreak was almost gone, with just 8,098 confirmed infections and 774 deaths worldwide.
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People were dying, and the World Health Organization issued a global health alert.īut this was 2003, not 2020, and the disease was SARS, not Covid-19. By March, the infection began to spread to other Asian countries and overseas. The cause, researchers would later learn, was a coronavirus new to science. The unusual cases of pneumonia began to appear in midwinter, in China.