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  • The sun is shining.

  • The birds are singing.

  • It looks like the start of another lovely day.

  • You're walking happily in the park, when, "Ah-choo!"

  • A passing stranger has expelled mucus and saliva from their mouth and nose.

  • You can feel the droplets of moisture land on your skin,

  • but what you can't feel are the thousands, or even millions,

  • of microscopic germs that have covertly traveled through the air

  • and onto your clothing, hands and face.

  • As gross as this scenario sounds,

  • it's actually very common for our bodies to be exposed to disease-causing germs,

  • and most of the time, it's not nearly as obvious.

  • Germs are found on almost every surface we come into contact with.

  • When we talk about germs,

  • we're actually referring to many different kinds of microscopic organisms,

  • including bacteria, fungi,

  • protozoa and viruses.

  • But what our germs all have in common is the ability to interact with our bodies

  • and change how we feel and function.

  • Scientists who study infectious diseases have wondered for decades

  • why it is that some of these germs are relatively harmless,

  • while others cause devastating effects and can sometimes be fatal.

  • We still haven't solved the entire puzzle,

  • but what we do know is that the harmfulness, or virulence,

  • of a germ is a result of evolution.

  • How can it be that the same evolutionary process

  • can produce germs that cause very different levels of harm?

  • The answer starts to become clear

  • if we think about a germ's mode of transmission,

  • which is the strategy it uses to get from one host to the next.

  • A common mode of transmission occurs through the air,

  • like the sneeze you just witnessed,

  • and one germ that uses this method is the rhinovirus,

  • which replicates in our upper airways,

  • and is responsible for up to half of all common colds.

  • Now, imagine that after the sneeze,

  • one of three hypothetical varieties of rhinovirus,

  • let's call them "too much," "too little," and "just right,"

  • has been lucky enough to land on you.

  • These viruses are hardwired to replicate,

  • but because of genetic differences, they will do so at different rates.

  • "Too much" multiplies very often,

  • making it very successful in the short run.

  • However, this success comes at a cost to you, the host.

  • A quickly replicating virus can cause more damage to your body,

  • making cold symptoms more severe.

  • If you're too sick to leave your home,

  • you don't give the virus any opportunities to jump to a new host.

  • And if the disease should kill you,

  • the virus' own life cycle will end along with yours.

  • "Too little," on the other hand, multiplies rarely

  • and causes you little harm in the process.

  • Although this leaves you healthy enough to interact with other potential hosts,

  • the lack of symptoms means you may not sneeze at all,

  • or if you do, there may be too few viruses in your mucus to infect anyone else.

  • Meanwhile, "just right" has been replicating quickly enough

  • to ensure that you're carrying sufficient amounts of the virus to spread

  • but not so often that you're too sick to get out of bed.

  • And in the end, it's the one that will be most successful

  • at transmitting itself to new hosts and giving rise to the next generation.

  • This describes what scientists call trade-off hypothesis.

  • First developed in the early 1980s,

  • it predicts that germs will evolve to maximize their overall success

  • by achieving a balance between replicating within a host,

  • which causes virulence, and transmission to a new host.

  • In the case of the rhinovirus,

  • the hypothesis predicts that its evolution will favor less virulent forms

  • because it relies on close contact to get to its next victim.

  • For the rhinovirus, a mobile host is a good host,

  • and indeed, that is what we see.

  • While most people experience a runny nose, coughing and sneezing,

  • the common cold is generally mild and only lasts about a week.

  • It would be great if the story ended there,

  • but germs use many other modes of transmission.

  • For example, the malaria parasite, plasmodium, is transmitted by mosquitoes.

  • Unlike the rhinovirus, it doesn't need us to be up and about,

  • and may even benefit from harming us

  • since a sick and immobile person is easier for mosquitoes to bite.

  • We would expect germs that depend less on host mobility,

  • like those transmitted by insects, water or food,

  • to cause more severe symptoms.

  • So, what can we do to reduce the harmfulness of infectious diseases?

  • Evolutionary biologist Dr. Paul Ewald

  • has suggested that we can actually direct their evolution

  • through simple disease-control methods.

  • By mosquito-proofing houses, establishing clean water systems,

  • or staying home when we get a cold,

  • we can obstruct the transmission strategies of harmful germs

  • while creating a greater dependence on host mobility.

  • So, while traditional methods of trying to eradicate germs

  • may only breed stronger ones in the long run,

  • this innovative approach of encouraging them to evolve milder forms

  • could be a win-win situation.

  • (Cough)

  • Well, for the most part.

The sun is shining.

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B1 US TED-Ed transmission germ sneeze sick common

【TED-Ed】How do germs spread (and why do they make us sick)? - Yannay Khaikin and Nicole Mideo

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    稲葉白兎 posted on 2015/03/14
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