The pervasive nature of viruses has had its impact throughout the living world. Viral disease can be found in plants, arthropods, protozoa and bacteria, but also in higher animals. Albeit more detailed knowledge of their biological, chemical and physical properties was mounted in the last 50 years, viruses are no modern phenomenon.
The identification of the causative agent of tobacco mosaic disease as a novel pathogen by the Dutch microbiologist Martinus Willem Beijerinck is now acknowledged as being the foundation of virology. However, as this was quite controversial considering the prevailing theories of the era, it took many years for virology to become firmly established as a discipline separate from bacteriology.
The development of the concept
Entities that we now know as viruses were initially acknowledged when it was found that certain pathogens are able to pass through filters meant to stop bacteria. In 1887, French chemist Louis Pasteur made this observation with rabies, unaware he encountered a virus at the time.
In 1886, German agricultural chemist Adolf Eduard Mayer discovered that tobacco mosaic disease could be transmitted to healthy plants if they are inoculated with extracts of sap from the leaves of diseased plants. Although he could not identify the pathogen, Mayer was certain that his microbe has to be a very unusual bacterium.
While, it is often cumbersome to assign a single date to the discovery of viruses, a Russian scientist Dmitrii Iosifovich Ivanovsky is given credit for first recognizing an entity that is filterable, submicroscopic in size and different from bacterium that might be the cause of the tobacco mosaic disease. He instituted the term “filterable agent” to describe such organisms before the term “virus” was embraced.
The filterable nature of tobacco mosaic virus was confirmed by Beijerinck, but his attempts to successfully isolate the virus were fruitless. In 1898, he proposed a theory of contagious living fluid and hinted that this agent can only reproduce within living cells. At the time, this idea was new and groundbreaking.
Therefore Mayer, Ivanofsky and Beijerinck each contributed to the development of a paradigm shift and a new concept: a filterable agent too small to be observed by light microscopy, but able to cause disease by multiplying in living cells. In 1898, Friedrich Loeffler and Paul Frosch described the first filterable agent from animals (the foot-and-mouth disease virus), while Walter Reed and his team in Cuba recognized the first human filterable virus – yellow fever virus.
At that time the term virus (derived from the Latin word for poison or slimy liquid) was used interchangeably for any infectious agent, thus it was applied to tobacco mosaic virus as well. Later this term became restricted in use to those agents that fulfilled the criteria developed by Mayer, Ivanofsky and Beijerinck, which were the first agents to cause a disease that could not be proved by using classical Koch’s postulates.
Progress in the field of virology
Once the concept of a filterable virus took hold, the experimental procedure was applied to many diseased tissues. By the end of the first quarter of the 20th century (which is practically the same 20-year timeframe that it took to establish the existence of viruses) more than 65 diseases of animals and humans had been attributed to these filterable agents.
Between 1948 and 1955, a critical transition converting animal virology into a laboratory science was a result of four important steps: overcoming the difficulty of culturing single cells, developing a line of cells from a cervical carcinoma (HeLa), developing an optimal medium for the culture of single cells (Eagle’s minimal essential medium), as well as demonstration of viral replication in a nonneuronal human explant of embryonic tissues.
In the modern era of virology (from 1960 onwards) the replicative cycles of viruses were described in great detail. Furthermore, virologists have demonstrated the elaborate interactions between viral genomes, viral proteins and the cellular machinery of the host. In the last 30 years, sampling, sequencing and computer technology provided the possibility to identify and sequence entire viral communities.
More than hundred years of virology have forged new concepts and provided novel insight into life. Launched by the work of the three aforementioned botanical scientists, foundations that galvanized the modern molecular era and ushered in the “golden age of vaccinology” were laid, leading directly to the prevention of measles, polio and other plagues of nature.