TMV: The First "Virus"

Shattering the Mosaic: How TMV Became Virology’s First Illusion

In the late 19th century, scientific medicine underwent a foundational shift—from the belief that miasma, or “bad air,” caused disease—to a germ-centered approach that focused on bacteria as the primary culprits. As bacterial research advanced, efforts to explain “infectious” disease centered on isolating specific microbes believed to cause specific illnesses. This shift began primarily in 1876 with Robert Koch’s work on anthrax, which—while accepted as the first “proven” case of bacterial causation—was still met with both logical and scientific criticism of his methods. Koch’s response to his critics led him to develop new techniques for obtaining pure cultures of bacteria while placing greater emphasis on the experimental demonstration of causation—both of which were missing from his anthrax work.

Recognizing the limits of mere association, Koch aimed to strengthen his claims by formalizing a repeatable, logically ordered framework designed to convince skeptics of the germ “theory.” Although he was not the first to propose causation criteria, Koch popularized a systematic approach—later known as Koch’s Postulates—which became the gold standard for establishing the microbiological etiology of disease. His postulates aimed to shift disease theory from speculation to testable, falsifiable hypotheses. For any microbe to be considered the causative agent of a disease, the evidence needed to satisfy all four postulates.

This was intended to ensure that the germ “theory” rested on a secure scientific foundation, not simply mere conjecture.

However, although Koch’s Postulates were widely accepted as the logical framework, it soon became clear that even when pure cultures were obtained, the criteria could not consistently be fulfilled. This failure undermined efforts to definitively prove bacteria as the cause of disease. Even Koch’s own research ran into contradictions: a bacterium might initially appear to fulfill the postulates, only to be later shown to violate one or more of them.

Beyond the difficulty in conclusively proving visible bacteria as causative agents through the fulfillment of the postulates, cases of disease began to emerge that could not be explained by microbes at all. In both human and plant pathology, phenomena were observed that eluded standard microbial detection—cases where no microbe could be found or reliably associated with the disease.

As it became evident that Koch’s Postulates had to be bent or ignored in order to claim that microbes caused disease—and that many diseases had no consistent microbial association—something had to give. When pure cultures failed to meet the criteria, a new theoretical entity was introduced: one that could explain disease without the burden of isolation or visibility. This pivotal shift was acknowledged in Principles of Virology:

“From these beginnings, investigation into the causes of infectious disease was placed on a secure scientific foundation, the first step toward rational treatment and ultimately control. Furthermore, during the last decade of the 19th century, failures of the paradigm that bacterial or fungal agents are responsible for all diseases led to the identification of a new class of infectious agents—submicroscopic pathogens that came to be called viruses.”

Fields Virology similarly admits that it was only when Koch’s Postulates—“an experimental method to be used in all situations”—broke down that the concept of the “virus” arose:

“By the end of the 19th century, these concepts became the dominant paradigm of medical microbiology. They outlined an experimental method to be used in all situations. It was only when these rules broke down and failed to yield a causative agent that the concept of a virus was born.”

These admissions reveal that the concept of the “virus” arose not from scientific success, but from failure—a desperate effort to preserve the causal framework of germ “theory” when its cornerstone could not be fulfilled.

One retrospective account candidly reflected:

“In Pasteur's day, and for many years after his death, the word "virus" was used to describe any cause of infectious disease. Many bacteriologists soon discovered the cause of numerous infections. However, some infections remained, many of them horrendous, for which no bacterial cause could be found. These agents were invisible and could only be grown in living animals. The discovery of viruses was the key that unlocked the door that withheld the secrets of the cause of these mysterious infections. And, although Koch's postulates could not be fulfilled for many of these infections, this did not stop the pioneer virologists from looking for viruses in infections for which no other cause could be found.”

This quote underscores what modern virology often obscures: the “virus” concept was born not from clear, causal demonstration, but as a placeholder for unknown causes due to the inability to satisfy Koch’s Postulates. It was a conceptual sleight of hand—preserving the appearance of scientific rigor while replacing evidence with inference.

With this new theoretical “pathogen”—an unseen, unisolated agent that escaped filters and evaded direct observation—scientists could continue to apply the germ “theory” framework in the face of contradictory evidence and the absence of visible microbes. It provided them with an explanation that maintained the illusion of causal certainty where none had been demonstrated.

This shift in explanation away from the logical framework of Koch’s Postulates gave rise to what would later become the first so-called “virus:” Tobacco Mosaic “Virus” (TMV). This case—though not involving human illness—played a pivotal role in shaping what the medical field came to accept as a “virus:” an invisible, theoretical “pathogen” invoked precisely when bacterial causation failed. As TMV is central to the emergence of the “virus” concept, it's time to examine in depth the key foundational evidence behind the “discovery” of the first “virus” and see whether it holds up under scrutiny.