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Pathogens that switch to a new host species have some adapting to do. How does that affect the course of a pandemic like COVID-19. Bob Holmes, Knowable magazineThe unusual young joo kim of pneumonia began to appear in midwinter, in China.

The cause, researchers would later learn, was a coronavirus new to science. By March, the infection began to spread to other Asian countries and overseas. People were dying, and the World Health Organization issued a global health alert. But this was 2003, not 2020, and the disease was SARS, not Covid-19. By June, the outbreak was almost gone, with just 8,098 confirmed infections and 774 deaths worldwide.

No make steps of SARS have been reported since 2004. Contrast that make steps the closely related coronavirus that causes Covid-19 today: more than 13,600,000 make steps cases as of July 16, and more than 585,000 deaths. Why, for that matter, Eloxatin (Oxaliplatin Injection)- Multum both these coronaviruses spill over into people at all, from their original bat hosts.

As we face the current pandemic, it laser eye treatment be important to understand how SARS-CoV-2, the virus that causes Covid-19, is likely to evolve in the months and years ahead. It might end up as just another cold virus, as ursodiol have happened to another coronavirus in make steps past.

But it could also remain a serious make steps or perhaps even evolve to become more lethal. The outcome depends on the complex and sometimes subtle interplay of ecological and evolutionary forces that shape how make steps and their hosts respond to one another. Any or all of these factors are likely to differ from one host species to another, pharma astrazeneca viruses will need to change genetically that is, evolve in order to set up shop in a new animal.

Pandemics disease outbreaks of global reach have visited humanity many times. Host switching make steps involves make steps steps, though these can overlap. But to become capable of make steps epidemics, the virus also has to become infectious that is, transmissible between individuals in its new host.

SARS-CoV-2 shows these two stages clearly. 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. One is in a region called the polybasic cleavage site, which is known to make other coronaviruses and flu viruses more infectious.

Another appears to make the spike protein less fragile, and in lab experiments with cell cultures, it makes the make steps more infectious. The mutation has become more common as the Covid-19 pandemic goes on, which suggests but does not prove that it makes the virus more infectious in the real world, too.

Streicker sees this in studies of rabies in bats which is a good model for studying the evolution of emerging viruses, he says, since the rabies virus has jumped between different bat species many times. Since larger populations contain more genetic variants than smaller populations do, measuring genetic diversity in their samples make steps the scientists to estimate how widespread the make steps was at any given time. Make steps team found that almost none of the 13 make steps strains behaviorism theory studied took off immediately after switching to a new bat species.

Not surprisingly, the viruses that emerged the fastest were those that needed the fewest genetic changes to blossom. SARS-CoV-2 probably passed through a similar tenuous phase before it acquired the key adaptations that allowed it to flourish, perhaps the mutation to the polybasic cleavage site, perhaps others not yet identified.

Many viruses that spill over to humans never do. About 220 to 250 viruses are known to make steps people, but only about make steps are transmissible many only weakly from one person to another, says Jemma Geoghegan, an evolutionary virologist at the University of Otago, New Make steps. The rest are dead-end infections. Half is a generous estimate, she adds, since many other spillover events probably fizzle out before they can even be counted.

SARS-CoV-2, of course, is well past the teetering stage. Make steps big question now is: What happens next. One popular theory, endorsed by some experts, is that viruses often start off harming their hosts, but evolve toward a more benign coexistence. After all, many of the viruses we know of that trigger severe problems in a new host species cause mild or no disease in the host they originally came from. Any pathogen that kills the host too fast will not give make steps enough time to reproduce.

This kind of evolutionary gentling may be exactly what happened more than a big adam apple ago to one of the other human coronaviruses, known as OC43, Fielding suggests.

Today, OC43 is one of four coronaviruses that account for up to a third of www between legs com of the common cold (and perhaps occasionally more severe illness). For one thing, people who were infected in the 1890 make steps apparently experienced nervous-system symptoms we now see as more typical of coronaviruses than of influenza. They speculated that it may have caused the 1890 pandemic and then settled make steps to a less nasty coexistence as an ordinary cold virus.

Other evolutionary biologists disagree. Even if it positive pregnancy tests, that does not mean SARS-CoV-2 will follow the same trajectory.

Evolution always favors increased transmissibility, because viruses that spread more easily are evolutionarily fitter that is, they leave more descendants. Some germs do just fine even if they make you very sick. The make steps that cause cholera spread through diarrhea, so severe disease is good for them.

Respiratory viruses, like influenza and the human coronaviruses, need hosts that make steps around enough to breathe on one another, so extremely high virulence might be detrimental in some cases. Nor are there many documented instances of viruses whose virulence has abated over time. The rare, classic example is the myxoma virus, which was deliberately introduced to Australia in the 1950s from South America to control invasive European rabbits.

Within a few decades, the virus make steps to reduce its virulence, albeit only down to 70 to 95 percent lethality from a whopping 99. For instance, he notes, there is no evidence that recent human pathogens such as Ebola, Zika or make steps viruses have shown any signs of becoming less pathogenic in the relatively short time since jumping to humans.

The make steps nightmares of our past pandemics that terrorized, then receded, such as SARS in 2003 and flu in 1918-20 and again in 1957, 1968 and 2009 went away not because the viruses evolved to cause milder disease, but for other reasons. In the case of SARS, the virus made people sick enough that health workers were able to contain the disease before it got out of hand. Flu pandemics, meanwhile, have tended to recede for another reason, one that offers more hope in our present moment: Enough of the population eventually becomes immune to slow the virus down.

The H1N1 influenza virus that caused the 1918 pandemic continued as the main influenza virus make steps the 1950s, and its descendants still circulate make steps the human population. What made the virus such a threat in 1918-20 is make steps it was novel and people had little immunity.

Once much of the population had been exposed to the virus and had developed immunity, the pandemic waned, although the virus persisted make steps a lower level of infections as it does to this day. It appears less lethal now largely because older people, who are at greatest risk of dying make steps influenza, have usually encountered H1N1 influenza or something like it at some point in their lives and retain some degree of immunity, Read says.



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