How do germs spread (and why do they make us sick)? – Yannay Khaikin and Nicole Mideo

How do germs spread (and why do they make us sick)? – Yannay Khaikin and Nicole Mideo


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.

63 thoughts on “How do germs spread (and why do they make us sick)? – Yannay Khaikin and Nicole Mideo

  1. I'm done with bears as doctors!
    He took one look at a collapsing girl and decided that she was a goner, no point exerting yourself. Call me speciest if you want, but I demand human doctors from now on! Or zebras.

  2. Get away from those bears little girl they're diseased.And by the looks of the one waving it's snot at you,and the one in the lab coat,I think they're testing a new weaponized form of whatever they got.

  3. what about antibacterial soap and all those things humans use to "fight" germs, all you're doing is killing the wildtype germs and leaving the ones with resistance (mutants) alive, allowing them to replicate and make it even worse for us. 

  4. What a capitalist of a bear, runs through the park, infecting people just so they need to see the doc, which happens to be him, so he can cash in on their sickness… oh and have you also noticed that he's into researching how to spread disease most efficiently?

  5. although I disagree with this method, it is why we are here today. plagues wipe out the weakest, leaving the fittest to repopulate with resistant genes. medicine prevents that. so down the line we are swimming against the current. exposing the whole world to HIV might yield a few males or females who resist it, if the repopulate, HIV is eradicated. this is all theory and might be quite flawed. and I hope I'm wrong about it. high infant mortality lead us to refine our genes to be more resistant. kids that die young due to common cold or flu, are weaker than their siblings who survive

  6. Hm, but I don't see that big of a difference with trying to eradicate them. If you try to eradicate them, there will be evolutionary pressures to be mild enough so that we don't care to treat it, or strong enough to survive the antibiotics. Similarly, if we stay at home and protect against the known modes of transmission, there will be evolutionary pressures to be mild enough so that we don't care and don't stay home, and to evolve cleverer and more robust ways of transmission that we didn't account for. Both this innovative form of transmission, and the increased resistance to antibiotics are equally undesirable. I guess the difference may probably come in the former treat being harder to evolve than the later, and/or being easier to treat.

  7. my roommate came home from chi-town with a cold. It was the worse cold I have ever had. Everyone in the house had a cough for a month. Wasnt even cold season either.

  8. the video didnt mention the fact that by being exposed to bacteria and viruses makes your body produce anti bodies. eating healthy has zero effects on our immune system because antibodies are produced from the stimuli of viruses. sometimes viruses can kill other viruses meanwhile others can pass unnoticed from your immune system and become part of your genetic genome. after all its no coincide our dna is 10% viral and our genetic genome is at least 60 to 80 percent viral. if we really want to be healthy its necessary that we expose our bodies to harmful substances so that our bodies can produce anti bodies over a course of time and then see what the idea of the strongest survive really means. dont be scared of being sick, enjoy it and slowly and maturely your body will become the equivalent of the united states military killing all germs and taking them directly to your colon for a cool down and then get rid of them

  9. Annoying how we get sick… I always get a cold at least once a year. Maybe it is due to the bad conditions I live in. Still, why do these viruses have to exist 🙁

  10. Wait if I drink out of a cup when I was sick and didn't wash it… and when I wasn't sick and drank out of the cup that still wasn't wash… Would I Get Sick Again???

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