Are the Viruses Cells?
Section 2.11. Posing this question is not an act of curiosity. It is an essential
question. The viruses really exist. They contain DNA or RNA
and proteins (in some of them are established lipids, carbohydrates and
other components), cause genetic recombinations and mutations as it is in
the normal cells, reproduce and create numerous generations, but up to
now they still have not found a place in the evolutionary tree of living
organisms (see Fig. 1–2).
The existence of viruses has been predicted long time before to be
really observed. For the doctors in ancient Rome “virus” has meant poison
of animal origin, and the diseases caused by such poisons has been called
“viral”. Also the monstrous disease hydrophobia has been assigned to
them. In the Renaissance it has become clear that one should make
difference between non-contagious diseases caused by poisons and
contagious diseases caused by infectious agents. The term “viral disease”
has remained only for the contagious ones.
In XIX century as a result of the advance in microbiology it has been
established that infectious agents are bacteria, which really cause many
contagious diseases. Because of that, they have been incorrectly
considered “morbid viruses”. At that time bacteriological methods have not
enabled to observe and identify real viruses, in order to differentiate from
bacteria. L. Pasteur has known that hydrophobia in man is caused by a
specific living agent and it is transferred by animals (for example through
biting by a dog), but he has not succeeded to cultivate and isolate it on any
bacterial medium.
At the end of the century it has been found out that these infectious
agents are smaller than bacteria, since they have passed through the
filters which normally have stopped the bacteria known at that time. For
that reason they have been called “filtering viruses”.
The beginning of the intensive study of viruses starts with revealing
the cause of the mosaic disease in tobacco (Nicotiana tabacum). In 1892
D. I. Ivanovsky has established that its agent is invisible under
microscope, passes through porcellanous filters and does not grow on
usual nutrient media. In 1915 F. Twort has discovered that the viruses
infect not only the eukaryotic cells of higher organisms, but also bacteria.
Two years later (1917) F. d’Hérelle who has arrived to analogical
conclusion called them “bacteriophages” (from bacteria and phagos —
eat. swallow). For brevity they are called phages, W. Stanley (1935) has
succeeded in obtaining the tobacco mosaic virus (TMV) in crystalline
form. The localization of TMV is given in Figure 2–85. The term “virus”
as “contagium vivum fluidum” is suggested by Beijerinck (1898).