Experimental proof of virus transmission and contagion

In my previous post, I addressed the myth that viruses have "never been isolated". Another favourite canard of the no-virus crowd is that "transmission of viruses has never been demonstrated."

Nothing could be further from the truth. The very discovery of viruses as such arose from observations of contagion and experimental transmission. The infectious agent in question could not be seen under the light microscope, and it could not be removed from the infectious material by filtration; its presence could only be ascertained from the reproducible experimental transmission and from the consistent and distinctive nature of the symptoms that arose in the experimentally infected animals or plants. This state of affairs was summarized in 1937 by the virologist Thomas Rivers wrote, in his essay Viruses and Koch's Postulates:

The exact nature of these agents is not known; some may be the midgets of the microbial universe, others may represent forms of life unfamiliar to us, while still others may be inanimate incitants of disease. Regardless of lack of complete knowledge of their nature, it is decidedly incorrect to say that these agents are unknown. The incitants of smallpox, vaccinia, poliomyelitis, yellow fever, fowl plague and tobacco mosaic are known; …

On to a few specific examples of viral transmission:

In my first post on the broader subject, I already discussed a study by Theil et al. on the the serial passage of a rotavirus-like virus in piglets. In another study, the same group of researchers describes the propagation of initially three viruses in piglets. One of these three viruses (let's call it A) was then isolated by the following approach: the virus specimen was diluted far enough to lose the least abundant virus. The piglets were infected with the remaining two viruses (B and C) and developed immunity against those. Reinfection with the original 3-virus mixture then resulted in the recovery of only virus A.

Aside from illustrating experimental infection and its prevention by immunity, this study also nicely shows how laborious virology can be if you can't use cell cultures. This is the reason why virologists will use cell cultures for propagating and isolating viruses whenever this is feasible.

Yellow fever is one of the most dangerous viral diseases known, and the story of how its mode of transmission was discovered in rather heroic experiments on human volunteers is widely known. It was described by the discoverers themselves already in the year 1900. The discovery led to the prompt elimination of yellow fever from Havana, Cuba and from the Panama Canal Zone under the direction of Colonel William Gorgas. He told this story in his book Sanitation in Panama, which also contains an interesting account of the malaria control measures in the Canal Zone.

Of particular interest to many is the airborne transmission of viral diseases. Some experimental studies on the airborne transmission of rabbit pox were reported by Westwood et al. and by Adams et al. The technique used in both cases was to put infected animals into one cage and uninfected ones into another, and then place the two cages near each other. Transmission between the cages was observed in both studies. Some digging in Pubmed will yield similar experimental observations with other viruses also.

This much on specific examples of studies viral transmission. A general recommendation: whenever one is looking for answers to scientific questions of a fundamental nature — such as: where did the idea of viruses come from? — it is useful to look at studies from the formative period of the discipline in question. And the easiest way to locate such foundational studies is to read review articles and books.

The first virus to be recognized was the tobacco mosaic virus. A good review on its history is Milestones in the research on tobacco mosaic virus by Harrison and Wilson(1999). A nice book on the history of virology in general is To Catch a Virus by Booss and August (2013 — and there is a newer edition from 2022). This book is broad in scope, thoroughly referenced and well written, even though a bit long at times on biographical detail. It will help you locate useful references on all manner of crucial ideas and findings.

With this short post, I plan to bow out of the “no virus” debate. I don’t think I have converted anyone from the “no virus” camp, but this was not the purpose. Instead, I have tried to present rational arguments for the benefit of those who may feel bewildered by this debate, but who approach this question with an open mind.