Historical background
The term "virus" comes from the Latin word for poison and was first used by Aulus Cornelius Celsus in the first century A.D.
However, there are much earlier documented cases of viral infections. The stone tablet shows an Egyptian from the 18th dynasty with polio (1580-1350 B.C.) – notice his withered leg and shape of the foot, which are typical of the disease.
Until the 17th century, the term virus was used to refer to all infectious diseases. No distinction was made between bacteria and viruses.
In the 17th century, Antoni van Leeuwenhoek invented the light microscope. Now, the term virus was used to describe only those "poisons" that could not be viewed through the microscope (i.e. < 200nm).
In the 19th century, germ theory was developed, which states that all infections are caused by a particular micro-organism, which must satisfy the following criteria:
- it can be grown;
- it is visible through a microscope;
- it can be retained by filters.
However, it was still thought that all transmittable diseases which did not meet these criteria were caused by poisons.
In 1892, Dimitri I. Ivanovsky showed that the agent behind the tobacco mosaic virus
- could not be cultivated on its own,
- was not visible,
- could permeate filters.
In 1898, Martinus Willem Beijerinck repeated this experiment and was convinced that this was a new phenomenon. He called this unknown agent "contagium vivum fluidum".
Also in 1898, Friedrich Loeffler and Paul Frosch observed that foot and mouth disease
spreads from animal to animal regardless of the concentration level of the agent, and that the duration of the disease was always the same.
They concluded that it was not caused by a poison, but something which reproduces itself.
As a result of all these discoveries, the field of virology was born.
