A VIRAL RETROSPECTIVE
As humans, we rely a lot on what we see, hear, smell, taste, and touch. It is through these senses that our responses are triggered into action. It’s not real unless you can sense it. We see a lion prowling around, we run. Did you hear that explosion? We should hide. Your food tastes bitter? Spit it out! Our senses help us to respond to our environment. But what happens when our senses fail us? When no matter how hard we try we simply cannot perceive what is out there? Luckily, we have our minds to guide us with scientific thought and examination.
The “germ theory of disease” that marked the beginning of a new way to think about infectious disease at the end of the 19th century can be summed up in four main principles:
1. The microbe must be present in all organisms suffering from the disease but not in healthy organisms.
2. The microbe must be isolated from a diseased organism and grown in pure culture.
3. The cultured microbe should cause disease when introduced into a healthy organism.
4. The microbe must be re-isolated from the new host and identified as being identical to the original causative agent.
Ultimately, this resulted in the miasma theory being debunked and effectively helped explain how bacterial diseases work. However, there were still some diseases that seemed to be bacteria-less even though they followed Koch’s postulates. One such example was Tobacco Mosaic Disease (TMD), an ailment that was causing major tobacco crop losses at the time. Late in the 1800s, agricultural chemist Adolf Mayer sought to determine the origin of TMD. Although Mayer was able to show that the disease was caused by an infectious agent, no matter how hard he tried, the bacterium remained elusive. He couldn’t even see it through a microscope! What on Earth could this be?!
A few years later, Russian biologist Dmitry Ivanovsky and Dutch microbiologist Martinus Beijerinck made two key observations.
First, they both confirmed that the agent was smaller than any known bacterium. Ivanovsky took the sap from infected leaves and passed it through a filter containing pores too small for bacteria to flow through. Had the infectious agent been a bacterium, the filtered fluid would have been sterile. However, the filtered fluid still caused TMD in healthy plants.
Second, they showed that the agent could only replicate within live cells, making it impossible to study. In attempts to distinguish this new type of infectious agent from bacteria, Beijerinck coined the term “virus”, describing it as a “contagium vividum fluidum” (contagious living fluid). TMD therefore became known as Tobacco Mosaic Virus (TMV), the first virus to be discovered.
full article in PLASMA 6 (coming June 2020)