Why the Independent Variable in Virology Remains Un-established
Virology rests on a foundational methodological flaw, one that has persisted since the very birth of the field, and it strikes at the core of scientific inquiry itself: how causation is established
The flaw is so severe that it erodes the very ground upon which virology stands. Exposing it completely dismantles the claims that sustain the discipline.
This is why the question of what it takes to scientifically demonstrate causation has remained central to my critique of virology.
However, before examining virology’s reliance on this error, we must first clarify what it actually means to demonstrate that something causes a specific outcome. Not merely suggest it. Not simply correlate with it.
But prove it as a true cause‑and‑effect relationship. That question lies at the heart of what makes a hypothesis scientific, and understanding this process makes virology’s fatal flaw impossible to overlook.
The Anatomy of a Hypothesis
Evaluating any claim of cause and effect requires a clear definition and understanding of what constitutes a scientific hypothesis. As defined by Britannica, a scientific hypothesis is a tentative explanation about a phenomenon or a narrow set of phenomena observed in the natural world.
At its core, it proposes that a specific, identifiable factor produces a particular outcome. But testing that proposal requires one essential step: the cause must be isolated and manipulated on its own, clearly distinct from alternative explanations.
To see how this works in practice, a scientific hypothesis begins with an observed natural phenomenon—an effect or outcome that occurs in the natural world and requires explanation. This observed outcome is known as the dependent variable (DV), the phenomenon the researcher seeks to understand.
For example, if a tree catches fire, the fire itself represents the observed outcome (DV) the researcher wants to explain.
The next step is to identify a proposed cause, known as the independent variable (IV). In this example, the hypothesis might propose that lightning striking the tree caused the fire. For this proposed cause to be scientifically tested, it must be identifiable, present, and capable of being independently controlled or manipulated to determine whether it can actually produce the observed effect.
In addition, all other possible causes, known as confounding variables (CV), must be accounted for. In this example, that could include a lit match, a discarded cigarette, or even environmental factors like heat buildup from dry leaves.
Only by isolating the proposed cause and controlling for alternative explanations can the researcher determine whether the effect is truly the result of that cause, rather than an assumption based on correlation or incomplete evidence.
This principle is reflected in the 2018 paper From Ideal to Real Risk: Philosophy of Causation Meets Risk Analysis. In section 4.2. Causation Must Be Studied by Comparison and in Isolation, it is emphasized that causal factors must be isolated or controlled in order to determine their specific contribution to an outcome.
The authors write:
“In the lab, one will try to isolate the causal factor as much as possible from its usual complexity to see what it does on its own, but this might not always be practically possible. RCTs are one way to do this, the idea being that if there is a difference in outcome between the two situations, it is because of the controlled factor. Only if the cause can be isolated or controlled in this way, can we say to have “observed” its causal contribution. Without such isolation, we cannot know if some other factor was present that produced the effect instead. This is called the problem of overdetermination and is a problem for studying causation in its natural complexity (Collins, Hall, & Paul, 2004). From an empiricist point of view, we thus see that causal complexity becomes a challenge for establishing what exactly the causal contribution of each factor is, which means that separation and isolation are necessary steps of the scientific procedure.”
The authors further acknowledge that individual factors must be distinguished from confounders if we are to determine whether they actually contribute to the outcome. This is one of the basic difficulties of establishing causation.
Natural systems are complex, and unless the variable being examined can be isolated, we cannot simply assume that the observed outcome was caused by that variable rather than something else.
This provides a straightforward standard for determining whether causation has actually been demonstrated. Observing an outcome and associating it with a proposed cause is not enough.
The cause must be identified and isolated, and shown to produce the outcome without relying on other possible explanations.
The Inversion of Logic
It is upon applying this scientific standard to “infectious” disease research that an important question is raised: does virology follow the basic rules of experimental science? Specifically, do researchers isolate and test the independent variable before observing its alleged effects?
This is where the fatal methodological flaws arise in virology.
While there is an observed phenomenon—illness and its apparent “spread”—and a clearly definable dependent variable in the form of symptoms, the corresponding independent variable is not established in a direct and controlled manner.
Examining the historical literature reveals that the concept of a “virus” did not arise as a discovered entity, but as a theoretical construct, a late 19th century placeholder explanation for disease processes that bacteria could not explain.
Unlike a lightning strike, which can be directly observed and tested independently of the fire it causes, the “virus” was inferred from the very effects it was proposed to explain.
Because the independent variable was not independently established and characterized apart from the effect it was claimed to produce, the foundational requirement for a true cause-and-effect demonstration was bypassed from the very start.
Instead, the effect—disease or cellular changes—is reproduced under artificial laboratory conditions using complex, unpurified materials such as fluids, tissues, or swabs. These materials are, by definition, a collection of confounding variables containing countless proteins, extracellular vesicles, cellular debris, and unknown factors.
When these are introduced into animals or cell cultures through unnatural routes of exposure, any observed effect cannot reliably be traced to a single, isolated variable. The cause is inferred from the effect rather than demonstrated as a distinct and manipulable variable capable of producing the effect on its own.
The requirement for a purified agent to prove causation is not some arbitrary requirement—it dates back to the mid-19th century. Yet throughout the birth of the field, researchers routinely acknowledged their inability to achieve full purification free of confounding factors.
While defenders of virology argue that strict physical isolation is unnecessary or unreflective of biological research standards, evaluating that claim requires examining the record itself: the standards established at the birth of the field, the admissions made across decades of literature, and where the methodology stands today.
You may be surprised to learn that, despite unprecedented technological advances, virology still operates without this critical missing variable required to truly demonstrate causation. The fatal flaw remains unresolved.
What follows is an intensive examination of this problem, broken into six parts within this article. Each part builds upon the last, tracing the historical development of the methodology, the attempts to establish the “virus” as an independent causal agent, and the problems that remain unsettled today.
It is a lengthy investigation, but one that requires time to fully examine and digest the evidence.
By the end, you will be asking: Where is the missing variable?
1. The Purity Standard
The Henle-Klebs Mandate
Decades before Koch made this statement, in the 1840 work Pathologische Untersuchungen, his teacher Jakob Henle emphasized the necessity of isolating a causal agent in order to study its effects independently.
He wrote that “one could prove empirically that [the organisms] were really effective only if one could isolate the contagious organisms from the contagious fluids, and then observe the powers of each separately.”
In other words, to ascribe causation to a specific microorganism, it must first be separated from all other potential causes of the same effect.
This principle was echoed in 1872 by the German-Swiss microbiologist Edwin Klebs, who noted that simply:
“…tracing the invasion and the course of the micro-organisms can make causality probable, but the crucial experiment is to isolate the efficient cause and allow it to operate on the organism.”
Together, Henle and Klebs established an important methodological principle that would become central to microbiology: the proposed cause (IV) must be separated from confounding factors (CV) so that its effects (DV) can be tested on its own.
Koch’s Early Concessions
However, this standard proved difficult to meet in practice during the early development of bacteriology. Throughout his work on anthrax in the mid-1870s, Robert Koch did not have a definitive method for ensuring complete purity of his cultures.
In his paper Robert Koch: From Obscurity to Glory to Fiasco, microbiologist Martin Dworkin pointed out that Koch’s anthrax work preceded the development of his plating technique for obtaining isolated colonies and was therefore conducted without the use of isolated pure cultures.
As historian K. Codell Carter noted in The Koch–Pasteur Dispute on Establishing the Cause of Anthrax, Koch’s reliance on microscopic observation to rule out contamination would not have satisfied many of his contemporaries, who regarded precise isolation techniques as essential.
In fact, Koch himself acknowledged the difficulty of meeting this standard. In his 1881 paper Zur Ätiologie des Milzbrandes, he described the demand to isolate bacilli from all associated substances as effectively “impossible,” remarking that “no one can take seriously such an undertaking.”
He further conceded that, because of the lack of purity, “one could still object that … some other substance associated with the bacteria, rather than the bacilli themselves, causes the disease.” Koch ultimately concluded that, for all practical purposes, the purity objection to his anthrax work was “meaningless.”
His contemporaries, however, did not share this view. As noted by Carter, critics of Koch’s work regarded this objection as fundamental. In reviewing Koch’s paper, Professor Zuber observed:
“How opinions can differ! We think, and much of the world with us, that this objection has a capital importance, and we reproach precisely those authors who…subject complex liquids to complicated procedures and thereby reach dubious results.”
Zuber went on to praise those who were attempting to completely isolate the bacillus through successive cultures, emphasizing the importance of achieving true separation from all other substances.
It was clear that Koch’s contemporaries understood the importance of isolating a single causal factor before attempting to establish causation. The presumed cause had to be separated from other influences so that its effects could be assessed independently.
This was precisely where Koch faced criticism: he had not fully demonstrated that the anthrax bacillus alone, independent of the other material involved in his experiments, was responsible for producing the disease.
For all intents and purposes, he had not yet conclusively established a causal relationship, and his evidence was regarded by many as insufficient.
The Methodological Pivot of 1881
Against this backdrop, Koch’s 1881 publication Zur Untersuchung von pathogenen Organismen marked a major shift in methodology. Having previously dismissed complete isolation as practically impossible, he now argued that isolating a single organism from all others was essential for proper study.
He wrote:
“And once it has been established that a notoriously harmful or even merely suspicious organism is present, the next task is to study it separately from others—which would otherwise interfere with and confuse observation—in all its characteristics: to investigate its conditions of life, its developmental history, and everything that promotes or inhibits it.
However, such knowledge can only be obtained with the help of continuous cultures of the individual species, so-called pure cultures, for which, up to now, no methods exist that are universally applicable and reliable in every respect.”
Here, Koch directly identified the problem of confounding variables (“others which would interfere”) and the necessity of isolating the proposed causal factor for testing.
To address this, Koch developed improved techniques for obtaining pure cultures using solid media (initially gelatin) on glass plates, allowing individual colonies of bacteria to be separated and propagated.
This process involves taking a mixed sample, distributing it across a solid surface to isolate individual colonies, and repeatedly subculturing a single colony to ensure purity. Koch later applied these methods in his work on tuberculosis, where he reported obtaining the tubercle bacillus in pure culture, consisting of a single species without admixture.
In his 1882 work Ueber die Milzbrandimpfung, Koch further emphasized that demonstrating causality requires freeing the organism from all diseased material before testing it:
“Only after one has, in this way, gained a thorough orientation as to whether microorganisms are present in the diseased parts, and at which locations they are found in complete purity—for example in the lung, spleen, heart blood, etc.—can one attempt to prove that these microorganisms are pathogenic in nature and that they are specifically the cause of the disease in question.
For this purpose they must be cultured pure and, after they have thereby been entirely freed from all the parts of the diseased body, they must be inoculated back into animals, preferably of the same species as those in which the disease was originally observed, or at least into such animals in which the disease in question is known by experience to occur with unmistakable symptoms.”
The Requirement of “Perfect Proof”
By his 1884 paper Die Aetiologie der Tuberculose, Koch went even further, stating that the only way to obtain “perfect proof” of causation was to completely separate the organism not only from the diseased host but also from all potentially confounding disease products:
“The facts obtained in this study may possibly be sufficient proof of the causal relationship, such that only the most sceptical can raise the objection that the discovered microorganism is not the cause but only an accompaniment of the disease. However, many times this objection has a certain validity, and then it is necessary to obtain a perfect proof to satisfy oneself that the parasite and the disease are not only correlated, but actually causally related, and that the parasite is the actual direct cause of the disease.
This can only be done by completely separating the parasite from the diseased organism, and from all of the products of the disease which could be ascribed to a disease-inducing influence, and then introducing the isolated parasite into healthy organisms and inducing the disease anew with all its characteristic symptoms and properties.”
Koch clearly distinguished between mere correlation and true causation, insisting that only complete separation of the organism from all other disease-associated material could establish a direct causal relationship.
Koch’s later emphasis on pure culture methods reinforced the need to isolate the organism from other potentially confounding material. He recognized that partial or uncertain separation would leave open the very objection that had been raised by his critics, that some other associated substance, rather than the organism itself, might be responsible for the observed effects.
In his attempts to address his critics, Robert Koch would later emphasize the central importance of pure culture techniques, writing that “the pure culture is the foundation for all research on infectious diseases.”
This principle remains foundational in modern microbiology. As the Journal of Visualized Experiments (JoVE) explains:
“Pure cultures, defined as the growth of a single microorganism species isolated from mixed populations, are fundamental tools in microbiological research…allowing researchers to study microbial traits under controlled conditions.”
Early bacteriology therefore treated isolation as a methodological requirement: the proposed causal agent had to be separated from a mixed population before its role in disease could be evaluated.
As microbiologist Martin Dworkin observed, Koch’s introduction of solid media for bacterial cultivation “allowed the emergence of pure culture microbiology” and, together with the later canonization of Koch’s Postulates, “reified an attitude about the microbial causation of disease.”
While achieving this in practice could be challenging, the underlying principle was clear. However, even as these advances were codified, the question of whether such isolation could be consistently achieved would soon be tested by the emergence of the concept of “filterable infectious agents,” where the definition of isolation would be forced to change entirely.
This is taken from a long document. Read the rest here substack.com
Header image: CNRS News