COVID-19: I Guess I Have To Talk About Koch’s Postulates

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

I can’t believe I have to talk about this. But I do. And I’ll try to make it quick.

My frustration with the COVID-denial narrative is palpable. Today I tweeted my quick take on their standard playbook:

I’ve tackled many, if not all, of these weak claims right here on this blog. I’m quite tired of talking about them, quite tired of having to convince people that a real thing is a real thing. I expected that my time would be most valued explaining some of the science underpinning the phenomenon we are presently experiencing, and not having to argue with strangers over basic logical fallacies.

Can we be done with this yet? Can we not just focus on getting the vaccines out as quickly as possible, so I can go back to full-time diaper changing and watching reruns of Stargate SG-1? Please stop with the conspiracy theories. They’re not helping, and they are genuinely hurting.

Apparently we’re not done with it. A colleague alerted me to another prominent COVID-denial tack that I had not really heard of. It goes something like this:

“Since COVID-19 does not satisfy Koch’s postulates, it is not caused by a virus, therefore the PCR tests are all false positives, there is no increase in mortality, and the whole pandemic is a scam.”

Sigh. Where to begin?

Let’s get the PCR test thing out of the way. I wrote a whole post explaining why the PCR tests are not finding a tsunami of false positives. You can read it here: “There Isn’t a PCR ‘Casedemic'”.

Okay, who is this Koch guy? Robert Koch was a 19th century microbiologist. In 1890, he and Friedrich Loeffler published four postulates to determine a causal relationship between a pathogen and a disease. Here are the postulates:

  1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
  2. The microorganism must be isolated from a diseased organism and grown in pure culture.
  3. The cultured microorganism should cause disease when introduced into a healthy organism.
  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.


First of all…. who gives a shit? Sorry, but these are not laws of nature handed down by the gods and Newton. They are a thought experiment put forth by a couple of guys at the dawn of the science of bacteriology as a guide for helping to diagnose diseases. This was an era before genetic testing, invasive scans, and electron microscopy. All scientists had to go on were “rules of thumb”.

If you’re going to cling to Koch’s Postulates as hard rules for modern medicine, why not adhere to other 19th century beliefs, like bloodletting and miasma theory? If you worship at the temple of Koch, I hope you also only travel by steam engine and refer to North America as “the colonies.” Consistency, people!

(Note: attempts have been made to update the postulates for modern usage, as per this article.)

Epidemiologists use guides like Koch’s Postulates to help our thinking, not to arrive at hard conclusions. A comparator is the Bradford Hill Criteria, which were put forth in 1965 as a set of postulates to establish medical causation when a randomized controlled trial is not possible. But even though we still make use of the Bradford Hill Criteria, no one pretends that they are infallible or complete.

Denialists apply Koch’s postulates to COVID-19 and find inconsistencies in the way the disease is discussed… because that’s what conspiracy theorists do.

Many years ago, I was invited onto what would later become the world’s most popular podcast —the Joe Rogan Experience— to debate someone who claimed that HIV is not the proximal cause of AIDS. Of course, he was basing his argument partly on Koch’s postulates because, again, that’s what conspiracy theorists do. It’s a quick way to sound scientific without actually having to deeply engage with the science. (By the way, I declined the invitation for a number of very rational reasons, and have regretted it to this day.)

I’m not the only one to choose to respond to the weirdness of using Koch’s Postulates to “disprove” the COVID pandemic. Surgeon/scientist “Orac” does a splendid job with “Quacks misuse Koch’s postulates to deny that SARS-CoV-2 causes COVID-19.” And the magnificent Dr Ian Mackay gets the job done with “Sigh, yes, the ‘COVID virus’ is real.”

If this topic matters to you, I highly recommend the two previous links. They are much more thorough than the slim arguments I am putting forth herein.

So let’s go through the postulates ourselves, shall we?


1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.

Even in Koch’s own time, this postulate was known to be false, or at least not always true. This is the basis of asymptomatic infection and viral load. Many people with cholera, in Koch’s time, were eventually found to have low levels of the causal bacterium, but did not cause actual illness. The same was true for typhoid. Koch himself retreated from the first postulate in his lifetime.

When it comes to COVID, indeed there is a strong correlation between disease severity and viral load. That observation in and of itself should be almost sufficient evidence to convince most people of a causal relationship between the SARS-Cov2 virus and COVID-19 disease. As well, increasing viral load is a predictor of whether you will die of COVID-19.


2. The microorganism must be isolated from a diseased organism and grown in pure culture.

Well this is mostly only relevant for bacteria and fungi and some parasites. It’s mostly not true for viruses, which cannot be grown in culture, depending on what you mean by that. Viruses effect illness by invading a host cell and reprogramming that cell’s genetics.

Viruses were not known in Koch’s time, so his postulates could not have been written to accommodate their nature.

One could, though, virally infect a cell culture and watch the infection propagate. The issue there is that virulence changes as that propagation takes place, as described in the paper, “Amending Koch’s postulates for viral disease: When “growth in pure culture” leads to a loss of virulence.”

So really, what can you conclude? Nothing.


3. The cultured microorganism should cause disease when introduced into a healthy organism.

Note the use of the word “should.” Koch was careful to phrase it thusly, because it is not always true. Many people with robust immune systems can become exposed to a variety of pathogens and not become diseased, Maybe you already have some immunity. Maybe you just have a robust constitution.

It is sufficient to note that a measurable proportion of people exposed to SARS-Cov2 go on to become sick with the disease COVID-19. We know they were exposed because they were exposed to people who themselves tested positive for the presence of the virus. The only way you can discount this is is if you believe the PCR test is detecting something other than the SARS-Cov2 virus. And, as I’ve noted, we’ve covered that already. PCR tests are extremely sensitive at detecting SARS-Cov2, and are also extremely specific in not mistakenly detecting other coronaviruses.

I should note that in multiple labs all over the world, SARS-Cov2 has been isolated. But only an insane person would then intentionally re-introduce the isolated virus to a healthy person. We know that some non-human animals, like the occasional dog, can indeed contract COVID. But I hope no one is intentionally seeking to infect dogs.


4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

What Koch is saying is that after making someone sick with COVID by exposing them to SARS-Cov2, you should then be able to detect SARS-Cov2 in that newly sick person.

The answer…. ding ding ding… is yes. This postulate holds. This is the very definition of secondary contacts in an epidemiologic contact tracing study. The index case becomes sick with COVID, tests positive for the virus, infects a contact, who then tests positive for the virus and subsequently becomes sick with COVID.

Are we clear now? If you accept that PCR tests detect the presence of SARS-Cov2 –which they do– then SARS-Cov2 satisfies Koch’s postulates as being the cause of COVID-19. Why are we even discussing this?


A Note on Mortality

Those pushing the Koch-doesn’t-apply narrative tend to also push the casedemic narrative, which then puts them in a quandary. If the disease isn’t real, then what’s killing all the people? The only way out of this intellectual mudslide is for them to argue that the people actually aren’t dying. Or that they’re not dying at unusual rates.

This is where a little mathematical acuity is dangerous without depth of subject area knowledge. Just because you can use a spreadsheet and know how to download public use data, it doesn’t mean you fully understand the ins and outs of computing excess deaths at the population level. I’ve seen a couple of amateur data scientists exclaim on social media that “their calculations” show that there is no increase in mortality, so it all must be a scam!

To do it right, some subtle statistical control must be applied, to account for shifts in age distribution, data biases, and reporting lags, and also an appreciation that full data do not arrive for months after the end of the year.

One US paper published last month accounted for some demographic shifts and concluded, “The COVID-19 pandemic resulted in an estimated 218 000 excess deaths in the United States between March and August 2020, and 80% of those deaths had COVID-19 as the underlying cause.” They are not the only ones to make that conclusion.



COVID-19 is real. It is caused by a virus called SARS-Cov2. The virus is detected by the PCR test. And people are dying of it at extraordinary rates. If you don’t believe me, then good luck with that cheap swamp land in Florida.

Frankly, I’m totally fed up with having to convince some individuals that real people are suffering with a real disease. Almost two million human beings have been killed by this thing worldwide in just one year. That’s almost two million grieving families whose loved ones perished in terror and agony and often isolation. So I really don’t have a lot of patience anymore for those who wish to discount that level of human suffering.