Articles, Blog

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

November 12, 2019

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.


  • Reply jack Johnson October 21, 2014 at 3:25 pm

    Great videoo!!

  • Reply Shilag October 21, 2014 at 3:30 pm

    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.

  • Reply SteveGottaGoFast October 21, 2014 at 3:31 pm

    can't wait to see creationsts' comments xD

  • Reply MZ October 21, 2014 at 3:34 pm

    Can't wait until we're living in space so we don't have to deal with this kind of thing…

  • Reply Mayara Vieira October 21, 2014 at 3:41 pm


  • Reply Graveage October 21, 2014 at 3:45 pm

    Cook mosquitoes before eating them. A high temperature will kill Malaria causing viruses.

  • Reply yourfullofsheite October 21, 2014 at 3:46 pm

    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.

  • Reply Javier C October 21, 2014 at 3:51 pm

    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. 

  • Reply Alumx October 21, 2014 at 3:59 pm

    Is this video implying that germs do not come from Germany?

  • Reply Jason Forbes October 21, 2014 at 4:11 pm

    All I learned was to stay away from germ sneezy bears in dense forests. .__.

  • Reply Duckie bee October 21, 2014 at 4:38 pm

    I wish people wouldn't sneeze on the back of my head on the bus…

  • Reply Vale Sarbu October 21, 2014 at 5:42 pm

    This goes to show that the doctors gave the pacient the germ (the bear is also the doctor)

  • Reply Kröte nschemel October 21, 2014 at 5:45 pm

    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?

  • Reply RK4000 October 21, 2014 at 5:50 pm

    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

  • Reply Sebastian Kierk October 21, 2014 at 5:58 pm

    This was so simplified that I don't actually feel like I learned anything at all.

  • Reply Duckie bee October 21, 2014 at 6:07 pm

    Is that a new voice?!

  • Reply Animesh Sharma October 21, 2014 at 6:17 pm

    aiming for the middle path 🙂

  • Reply FNHot October 21, 2014 at 9:03 pm


  • Reply After Effects Tutorials by MJake October 21, 2014 at 9:53 pm

    aaaand I'm done with dinner. 

  • Reply Aerobrake October 22, 2014 at 4:06 am

    0:36 weird!

  • Reply Sonny Darvishzadeh October 22, 2014 at 6:40 am

    that fucking bear

  • Reply Olivier Rovelli October 22, 2014 at 12:07 pm

    I've always wondered about all of this works !

  • Reply Winny Han October 24, 2014 at 3:02 pm

    Lesson from this video: Stay away from fucking bears!!!!

  • Reply Gamer_Shrimp 88 October 25, 2014 at 3:38 pm

    Goldilocks & the 3 bears explaining germs ahahah 😀

  • Reply Daniel Mbaabu October 28, 2014 at 12:28 pm

    do bears sneeze in the woods?

  • Reply William Herring November 14, 2014 at 3:19 pm

    That's one nasty bear, he needs to be put down.

  • Reply ibrahim najjar November 16, 2014 at 9:58 am

    whats with the pedobear?

  • Reply Fatima Prieto November 18, 2014 at 11:38 pm

    Ahhh uf school yupi docente ve tus ahhhh fecundar frabnstdvgk

  • Reply Heidi Peterson November 19, 2014 at 4:10 pm

    Mabey good

  • Reply Mike Wazowski November 19, 2014 at 11:40 pm


  • Reply Guillermo Valle January 10, 2015 at 7:06 pm

    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.

  • Reply Abby W March 19, 2015 at 6:17 pm

    Go EEB

  • Reply Vaibhav ramani July 26, 2015 at 5:33 am

    How do you make these videos???

  • Reply rukhsana ground December 1, 2015 at 7:39 pm

    get to the point.

  • Reply Leafy Son December 5, 2015 at 12:45 pm

    I don't mind carrying a virus that doesn't do any harm to me or others.

  • Reply slickology April 25, 2016 at 5:45 pm

    31 germs dislike this video

  • Reply Oliver Joyce April 30, 2016 at 11:31 am

    who just clicked on this video

  • Reply Jonathan Schossig May 27, 2016 at 12:14 pm

    Bear comments below

  • Reply Lucy yyy August 9, 2016 at 6:52 am

    And this is why I'm afraid of germs

  • Reply Sarz October 16, 2016 at 1:57 pm

    Goldielocks and the three bears?

    ah, the nostalgia

  • Reply ALI LOL productions December 1, 2016 at 2:22 am

    this helped

  • Reply Stephanie Boyle April 4, 2017 at 4:20 pm

    Super Eurocentric

  • Reply Pulkit thareja July 15, 2017 at 3:10 pm

    knowledgeable for me

  • Reply Sophie_the_Cat03 August 25, 2017 at 2:26 pm

    As a germaphobic I diedddddddd

  • Reply M M September 18, 2017 at 4:19 am


  • Reply สยุมพร อินทมาน November 2, 2017 at 3:15 am


  • Reply MIke M November 4, 2017 at 6:03 am

    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.

  • Reply kirsten laska December 10, 2017 at 6:17 am

    I feel like I'm going to puke

  • Reply Conchaquat March 20, 2018 at 9:15 am

    The Comments here are funny.

  • Reply Luke Krier March 23, 2018 at 9:31 pm

    RIP golilocks 🙁

  • Reply SUPER SAMMY CHANNEL September 19, 2018 at 2:29 am


  • Reply Emilios Powerballer November 23, 2018 at 12:07 am

    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

  • Reply Drop Jaw Flies January 10, 2019 at 11:53 pm

    Evolution???? ………What a joke !

  • Reply pond Dude January 13, 2019 at 2:58 pm

    She shouldn't seen close to that bear anyways

  • Reply Liam Manning January 14, 2019 at 10:30 pm


  • Reply Liam Manning January 14, 2019 at 10:31 pm

    lol this taught me nothing

  • Reply Liam Manning January 14, 2019 at 10:34 pm

    henry has big gey

  • Reply Tiryn Quinn January 14, 2019 at 10:43 pm

    your mom has great big peen

  • Reply meghna sankhla March 5, 2019 at 1:18 pm

    And here we are scolded if we take an off due to a cold as we are supposed to “bear it”.

  • Reply Jack Michaud September 11, 2019 at 1:10 pm

    school work time

  • Reply Samrocket132 Fitsjerols September 18, 2019 at 12:58 am

    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???

  • Reply Amo Ranimo October 8, 2019 at 7:26 pm


  • Reply Hojin Chang October 18, 2019 at 7:39 pm


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