Articles, Blog

How Studying Bacteria Almost Kept Us From Discovering the Flu

September 8, 2019

{♫Intro♫} If you’re coughing, sneezing, or starting
to feel under the weather, you might blame a virus, or possibly a bacterium. Which is not something humans have known to
do for very long. Around 400 B.C.E., doctors might have blamed
an imbalance of the four vital humors for your illness. Around the 1700s, they might have pointed
to an invisible, disease-carrying fog instead. Today, we know pathogens — viruses, bacteria,
and certain other microbes — are responsible for many diseases. But linking specific diseases to the microbes
that cause them has been surprisingly tricky. In 1882, a scientist named Robert Koch demonstrated
that the bacterium Mycobacterium tuberculosis causes tuberculosis. And in 1890, he also published a framework
for future scientists to make similar discoveries. He created a checklist for researchers to
reference any time they’re trying to link a pathogen to a disease. The steps are as follows. First, researchers had to be able to find
the pathogen in sick organisms, but not healthy ones. Second, it could be grown in pure culture
— which means that a sample of the microbe could be taken from a sick organism, and then
the microbe could grow independently in a 19th century version of the petri dish. Third, if they exposed a healthy organism
to the stuff that they grew in step two, that organism would get sick with the same disease. Finally, though this step is sometimes considered
optional, the same microbe that was isolated in step one must be found again in the organism
made sick in step three. These steps are now known as Koch’s Postulates. The idea is that if the microbe meets all
of the postulates, then you know it’s the cause of the disease. Unfortunately, his postulates had a few problems. Take postulate one. Tuberculosis can actually be found in healthy
individuals — that’s called latent tuberculosis — so it doesn’t meet Koch’s first postulate. This situation just didn’t show up in his
experiments, which were done in guinea pigs. Postulate three isn’t perfect either. Assuming that any healthy organism exposed
to a pathogen will get sick ignores differences in immune systems. A healthy organism might be able to fight
off the infection or might already be immune to the disease. But it was the second postulate that caused
the most confusion. Something grown in “pure culture” has
to be the only living thing in the dish, and many pathogens just can’t grow independently
like that. Viruses, for example, reproduce by hijacking
molecular machinery in the cells of the organism they’re infecting. Meaning you can’t grow them in a dish by
themselves. But bacteria often grow in a dish just fine. Because postulate two required the thing to
grow in culture, researchers at the turn of the twentieth century would almost exclusively
blame bacteria for the diseases they were studying, which resulted in some false accusations. Malaria, which is actually caused by blood-infecting
parasites, was blamed on a bacterium from Italian marshes in the 1880s, which they named
Bacillus malariae. Canine distemper, a sometimes deadly disease
in dogs that causes symptoms like fever and vomiting, was linked to a series of different
bacteria before it was finally proven to be a virus in the 1920s. And the familiar virus influenza, or the flu,
was misidentified as a bacterium in eighteen ninety-two, by a colleague of Koch’s. The bacterium came to be known as Haemophilus
influenzae. To study the flu, researchers needed samples
of spit and snot from people with obvious symptoms. But one thing that made influenza hard to
study was that, even though the flu usually reaches a peak in winter, the only time that
scientists could reliably find large numbers of flu-ridden folks at the same time was during
a pandemic. And those could be decades apart. So the first chance scientists had to check
the results from 1892 was during the next influenza pandemic… in 1918. Researchers were unable to replicate those
initial results. But it wasn’t clear at the time if it was
because of poorly controlled studies in the chaos of one of the worst pandemics in recent
history and the end of World War I, or if they were just… wrong. A vaccine was developed in New York based
on Haemophilus, just in case. There was at least one study around that time
that managed to find evidence of the right answer: influenza is a virus. It took until 1933 and another influenza pandemic
for scientists to prove without a doubt that the flu is caused by a virus, thanks to the
introduction of ferrets as a model organism. Ferrets were the only small mammals they could
find that actually get the flu and show symptoms similar to ours. So it seems like Koch’s Postulates, especially
the second, really hindered research into any disease that didn’t have a bacterium
behind it. Does that mean they’re useless? Not at all. Since the 1880s, scientists have tweaked Koch’s
postulates over time to match modern understandings of pathogens. Today, the focus isn’t just on microbes,
but on their genes. Using genetic sequencing, scientists can gather
information about all of the nucleic acids in a sample, whether DNA or RNA, and then
use a modified version of Koch’s postulates to figure out which genes are most associated
with disease symptoms. For example, in 1996 scientists at Stanford
came up with a new set of postulates with seven gene-centric points. By using gene sequencing, scientists can find
pathogens that haven’t been isolated and identified before. And there’s no need to culture them. Koch’s postulates provided a solid foundation
for researchers to begin linking diseases to their sources. Sure, there were a few mistakes, but they
provided a rigorous, testable basis for understanding disease. Even if we had to come along and make some
changes later. And even if some ferrets had to get the sniffles. Thanks for watching this episode of SciShow,
and thanks to our supporters on Patreon for making what we do possible. If you want to join them and help us, while
also getting some cool benefits, check out {♫Outro♫}


  • Reply thekraken July 29, 2019 at 11:37 pm


  • Reply roblox master July 29, 2019 at 11:37 pm


  • Reply Nalayer July 29, 2019 at 11:37 pm


  • Reply Bret Bates July 29, 2019 at 11:37 pm


  • Reply Robo Slayer July 29, 2019 at 11:37 pm


  • Reply Just Some Bigfoot With Internet Access July 29, 2019 at 11:37 pm

    Luckily we bigfoots have a lower risk of getting the flu out here in the woods

  • Reply Ratko Mladic July 29, 2019 at 11:39 pm

    You nerds study bacteria? How much would you pay for a nice fat bag of homebrew?

  • Reply Eric Knijnenburg July 29, 2019 at 11:39 pm

    Last time I was this early bacterial antibiotic resistance was still rare

  • Reply Justin Y. July 29, 2019 at 11:39 pm


  • Reply Samuel Xu July 29, 2019 at 11:39 pm

    Justin Y. you are EVERYWHERE.

  • Reply Samuel Xu July 29, 2019 at 11:41 pm


  • Reply aliLikespie7 July 29, 2019 at 11:42 pm


  • Reply LIST July 29, 2019 at 11:42 pm

    Help me,i don't now where i am, i've been kidnaped in a basement by Duolingo bird, he doesn't now yet i've send this message, please hurry and rescue me, i don't want to die.

  • Reply Avery the Cuban-American July 29, 2019 at 11:43 pm

    Bacteria! We begin with only one. Bacteria! Two is what we then become. Bacteria! Each of us becomes two more. Bacteria! We are stronger than before. Bacteria! We keep growing at this rate. Bacteria! No longer shall we wait. Bacteria! The plan now unfolds. Bacteria! We will take over the world!

  • Reply Nodd Mouseson July 29, 2019 at 11:47 pm

    Scientists back in the day just didn't know how to deal with something going viral.

  • Reply Fruit Connoisseur July 29, 2019 at 11:47 pm

    Im so early the birds are not awake yet

  • Reply Outship July 29, 2019 at 11:47 pm


  • Reply Isaiah Hudson July 29, 2019 at 11:48 pm

    We have been trapped by duolingo bird

  • Reply Roy July 29, 2019 at 11:50 pm

    Made me sad to watch the sick dog. Poor thing.

  • Reply Matt Calza July 29, 2019 at 11:52 pm

    Eight views, 3 likes, and 83 comments. WTF?!?!

  • Reply Tim Travasos July 29, 2019 at 11:54 pm


  • Reply John Opalko July 29, 2019 at 11:56 pm

    That's how H. influenzae got its name! I always wondered about that. Thanks!

  • Reply B July 29, 2019 at 11:56 pm

    ahhh that haemophilus pronunciation

  • Reply Matthew Madruga July 30, 2019 at 12:11 am

    For a while…. Its way too big for us to not pick it up. We got cancer which was called witchcraft!

  • Reply Aidan Hauser July 30, 2019 at 12:14 am

    hank daddy do orthotropics

  • Reply Taker 314 July 30, 2019 at 12:16 am

    It's pronounced /kɔx/, not /koʊk/

  • Reply Master Therion July 30, 2019 at 12:20 am

    It seems like I "discover" the flu every winter 🙁

  • Reply SeanNicholasMusic July 30, 2019 at 12:28 am

    I am a ferret

  • Reply West Shed July 30, 2019 at 12:33 am

    I think I need my flu vaccine.

  • Reply Pendulunium240 July 30, 2019 at 12:34 am

    News: too few antibiotics in pipeline to tackle global drug-resistance crisis, WHO warns

    Bacteria: /watch?v=Zq9gy6MFrl4

  • Reply ThatYoutube channel July 30, 2019 at 12:39 am


  • Reply DKLUM July 30, 2019 at 12:43 am

    Scishow might be haunted, there's an echo of Stefan or is that me?

  • Reply Drāno drain cleaner July 30, 2019 at 12:47 am

    Poor ferets flu is no joke man

  • Reply danny obrian July 30, 2019 at 12:53 am

    Did you have to show a picture of a mosquito ! ! Dammit I'm itching and can't sleep now meeeeeeeeeeee splat ! Now I can

  • Reply Flying Spaghetti Monster July 30, 2019 at 12:57 am

    I feel so bad for that dog!

  • Reply Christel Headington July 30, 2019 at 12:58 am

    How clicking this on notifications almost kept me from discovering the video: (When I clicked it YT told me is was not available.)

  • Reply Triforce wielder July 30, 2019 at 12:58 am

    Oh, Arthur ?

  • Reply Stephan Zielinski July 30, 2019 at 1:04 am

    Note that Robert Koch is also infamous for attempting to become a gazillionaire off a cure for tuberculosis that didn't work, and sometimes even caused the latent form to become active. See

  • Reply Just Mavi July 30, 2019 at 1:05 am

    I feel like the video of a dying dog was a bit unnecessary..

  • Reply contrarian duude July 30, 2019 at 1:24 am

    If I'm not planning on any flights do I still need a flu vaccine? And do I have to wait until I flu?

  • Reply Diseasel July 30, 2019 at 1:24 am

    You know it's not really pronounced "coke" right?

  • Reply David barnes stuff 2 July 30, 2019 at 1:36 am

    I'm willing to bet that most people who think they have had the flu, have never had the flu. I have no evidence for this and since this is just a stupid YouTube comment I don't care.

  • Reply Michael Freed July 30, 2019 at 1:40 am

    The Flu Pandemic of 1918 they said?

  • Reply Tiffany Murfitt July 30, 2019 at 1:42 am

    The video of the dog isn't necessarily dying. It could be just lying there being weird, as dogs are known to do. But the photo of the other dog being microchipped is weird. It seems to me that images with syringes of any sort are inserted into montages kind of randomly. There are differences between vaccines, heroin use, blood sampling, and insulin use, people! Now add microchip insertion to that list.

  • Reply L M July 30, 2019 at 1:53 am

    Pff, well look at the name! I could tell you that mycobacterium tuberculosis causes tuberculosis!

  • Reply THeDoMeTB July 30, 2019 at 1:55 am

    Excuse me, who was that tuberculosis guy? Coke?
    it seems like you didnt even try to pronounce Koch right, and that hurts my little german heart 🙁

  • Reply Anne Duncan July 30, 2019 at 2:09 am

    19th century version of a petri dish? you know koch also invented the very idea of petri dishes?

  • Reply Osmosis Jones July 30, 2019 at 2:19 am

    Poison glass's can St make you sick

  • Reply cody hedges July 30, 2019 at 2:20 am

    If I'm sick, I blame witches!

  • Reply Josiah Hockenberry July 30, 2019 at 3:02 am

    I'm sure those ferrets didn't just get "the sniffles".

  • Reply Danielle McVay July 30, 2019 at 3:22 am

    I remember learning about Koch's postulates inn my first microbiology class is only it was just ferrets that got the sniffles! So much animal death! But also he made huge strides is microbial knowledge

  • Reply Psean's Vlog's July 30, 2019 at 3:41 am

    Question how to hold your poo? Hurryy plsss

  • Reply Celu The Fox July 30, 2019 at 4:00 am

    Not the ferrets! 🙁

  • Reply shiny x July 30, 2019 at 4:20 am

    I bet a ferret with the flu is a lot more adorable than a human with the flu. Atchoo!

  • Reply zedirich7 July 30, 2019 at 4:24 am

    The poor ferrets!

  • Reply Rishabh Bharadwaj July 30, 2019 at 4:24 am

    Is it OK to use the same trimmer/scissors for facial hairs and pubes?
    Just curious to know! ?

  • Reply Josh C July 30, 2019 at 4:34 am

    2:39 WHY would you show this!?? wasn't prepared

  • Reply Muscle Hank July 30, 2019 at 4:35 am

    What were the bacteria studying though

  • Reply Solon Simpson July 30, 2019 at 5:03 am

    What is the new set of postulates? I've got science blue balls over here.

  • Reply GlowingSnow28 July 30, 2019 at 5:10 am

    ''…The idea has been around for years, genome as data. You see once we digitized dna we made it infinitely mutable. We could do a thousand virtual variations in the time it use to take to grow a one lab grade generation…''

  • Reply tailsofpearls July 30, 2019 at 7:11 am

    If y'all are going to say koch's name so many times at least take the two seconds it takes to say it correctly!!! My god I didn't think it was possible to mispronounce it that badly!!!

  • Reply Isaiah Dobesh July 30, 2019 at 8:10 am

    Am I the only one hoping the dog at 2:43 is ok and just in the middle of dreaming?

  • Reply Crypted Frost July 30, 2019 at 8:17 am

    Who else didn't know that "Studying Bacteria" was a phenomenon!

  • Reply jogewe July 30, 2019 at 8:25 am

    That's not how you pronounce Koch

  • Reply FestiveParrot July 30, 2019 at 9:31 am

    … hoped i got to see hank ?

  • Reply TheDroidBay July 30, 2019 at 10:06 am

    Pretty sure you don't pronounce his name as "coke", but then the german pronunciation would have lead to a certain predictable variety of comment, and I'd guess, people not paying attention due to sniggering.

  • Reply KerriNature July 30, 2019 at 11:12 am

    Afternoon from Canada 🙂 Sadly the ferrets got more than the sniffles. Just a side note then I've found over the years. I had ferrets for a brief time and that's when I learned about their use in medicine. From what I could figure out, the ones in North America are largely from the same company. Sadly this shows in the health of the animals. Just the state of things from being in demand and from what I also understand, kind of hard to breed animals. Where I'm from, all ferrets are neutered before sale. I hope this is old information and things have improved for them or what I read was wrong, but such a small genetic pool just does things like that I think. The ones overseas are much healthier from what I hear, they're larger and able to live outdoors. Wonderful animals though, so funny with their little quirks of stashing things they find. tunnelling through anything they can, chasing and doing funny war dances as they bounce around making the most adorable sounds. Demanding little pets so you want to be sure about sharing life with them, also sad they don't live long.

  • Reply Aerol Christopher Infante July 30, 2019 at 1:20 pm

    Whew. I also see the link of MYCOBACTERIUM TUBERCULOSIS is the cause of tuberculosis.

  • Reply Gareth Dean July 30, 2019 at 1:30 pm

    Koch's postulates are a perfect example of ivory tower thinking. In a perfect world they'd be brilliant at sorting the actual from the possible causes of disease.But in the real world they're barely good as a guide and often unethical. (Anyone want to deliberately infect someone with HIV? No? But the postulates!)

  • Reply Ben U July 30, 2019 at 2:02 pm

    Hey guys, I noticed you don't have a dedicated video about modern day Human testing (drugs, implants and the like) yet. Seems like that would be interesting, in regards to "should you consider taking random drugs as an income source or not?".

  • Reply Dragon Skunk Studio July 30, 2019 at 2:04 pm

    2:39 sneezy doggo

  • Reply Douggernaut84 July 30, 2019 at 2:41 pm

    Those flu ferrets got the sniffles.. then got dissected.

  • Reply Herr Schmidt July 30, 2019 at 3:40 pm

    it's Koch not Coke dammit D:

  • Reply klutterkicker July 30, 2019 at 4:16 pm

    What those steps do well is prove that the presentation of a disease is linked to an organism that goes through its life cycle jumping from one host to the next. But other than the symptoms mentioned, the system can be tripped up by diseases that have similar symptoms, since it can be difficult to prove that two people even have the same disease.

  • Reply lejoe2laglace July 30, 2019 at 8:25 pm

    Robert Coke 🙂

  • Reply deitylink1 July 30, 2019 at 8:38 pm

    BCE is for tools. It clarifies nothing, and only obfuscates what is being talked about, merely because you don't like the event around which the calendar orients

  • Reply Gustav Gnöttgen July 30, 2019 at 8:59 pm

    4:58 ferrets and sniffles!
    Do you suffer from Sam O'Nella?

  • Reply Victoria July 30, 2019 at 9:22 pm

    You know, the fog was actually closer to the truth as far as diseases are concerned than the humors theory.

    I mean, lots of things can be spread through the air, but less blood is basically never the answer.
    I don't even get how they managed to think bleeding was a good idea for so long. It's pretty obvious that if you lose too much blood you die. And if your cuts get infected you die. And if you have blood coming out of basically any or orifice, you probably (back then) are going to die. All signs point towards blood outside the body being a bad thing.
    I just don't get it.

  • Reply Yellow Penetrator July 30, 2019 at 9:38 pm

    Id say initially instead of almost

  • Reply GiggitySam Entz July 30, 2019 at 9:56 pm

    "Robert Coke" XD
    Apparently in English it's pronounced "Kok" but it sounds weird to be, in German it's closer to "Kohh"

  • Reply Burniture July 30, 2019 at 10:28 pm

    Is there an easy way to tell apart damage caused by infection from damage caused by toxic substances (like compounds or certain elements), or is it just hit and miss?

  • Reply Tim Porwoll July 31, 2019 at 12:26 am

    i am sorry, but as a german i feel kinda weirded out when hearing the name "koch". the "ch" isn t pronounced as a "g". i can t think of an english equivalent but there is probably one.

  • Reply Christian Kleinbub July 31, 2019 at 2:51 am

    Just wondering–don't know where to put this question–but would Scishow address this question at some point: what happens when one animal swallows another animal whole? How does it die? Does it suffocate on the way to the stomach or does it somehow just burn away in acid?

  • Reply Number Eight or Nine? July 31, 2019 at 3:09 am

    are we in an age where people no longer strive to improve upon the things we already have?

  • Reply MrMegaPussyPlayer July 31, 2019 at 4:08 am

    4:27 Unless it is something that hasn't DNS … then it useless again

  • Reply Ken O July 31, 2019 at 7:44 am

    Saying "the sniffles" seems like it might denigrate the suffering and deaths of all people who've gotten the flu. HOWEVER, what if ferrets actually ONLY get the sniffles and not much more, when contracting the flu. I looked it up, and found: Unless the ferret develops secondary bacterial infections of the sinuses or pneumonia, the disease is usually self limiting and ferrets will recover in 7–10 days. Still probably more than just the sniffles.

  • Reply Zes July 31, 2019 at 9:11 am


  • Reply insertnamehere001 July 31, 2019 at 4:01 pm

    Was Koch's collegue named Heckler?

  • Reply deepsy2k July 31, 2019 at 6:25 pm

    apparently, the guinea pigs were actually used as guinea pigs (⓿_⓿)

  • Reply Chris Boucher July 31, 2019 at 11:00 pm

    Stephan, so interesting.

  • Reply VwertIX August 2, 2019 at 7:03 pm


  • Reply Nick Walker August 3, 2019 at 4:26 am


    They act wirey like me.
    They cough like me.
    They lose my damn car keys like me…
    Therefore I must be Ferret. 😐

  • Reply Nmethyltransferase August 19, 2019 at 1:48 am

    laughs in Bradford Hill Criteria

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