current speculation that some cancers may be caused by a virus. Some viruses cause
the development of inclusion bodies in the cells they attack. These inclusion bodies
may be colonies of the virus, or they may be products formed by the cell in response to
the attack. The presence of inclusion bodies is very important in the diagnosis of
certain diseases where it is not possible to isolate the virus. In rabies, for example, the
presence of Negri bodies (named for their discoverer) in brain cells has been used to
confirm the diagnosis, even though the rabies virus may not be isolated.
Certain viruses have the ability to infect bacterial cells. Such a virus is known as
a bacteriophage ("bacteria eater"). Bacteriophages are lunar-lander shaped
(figure 2-7(E)) viruses, the action of which is specific for any given species. In other
words, coliphages attack only coliform bacteria; staphylophages attack only
staphylococci; and so on. Very few bacteria (pneumococci are an example) do not
a. Action of Bacteriophages. The action of bacteriophage on a bacterial. cell
is shown in figure 2-8. The virus attaches itself to the bacterial cell (figure 2-8 A),
whereupon the tail penetrates the cell wall by chemical action. The DNA of the virus
then flows into the bacterial cell (figure 2-8 B). At this point with some viruses the viral
DNA joins (links) with the bacterial DNA. Now, whatever genetic code is carried on, the
viral DNA will be translated into proteins. Many times these proteins are powerful
exotoxins or destructive enzymes. If the viral DNA remains separate from the bacterial
DNA then the DNA takes over the action of the cell, synthesizing hundreds of new
viruses. When the numerous new phage particles have formed, the cell structure
disintegrates, liberating the newly formed virus particles (figure 2-8 C).
b. Importance of Bacteriophages. Bacterial viruses occur in nature with their
specific hosts. Their highly specific nature assists the bacteriologist in classifying and
typing bacteria (an additional identification technique, known as phage typing).
Bacteriophages are not always beneficial; in certain industries that depend upon
virus could cause great financial loss. Many of the pathogenic effects of bacteria
(enzymes and toxins) are produced only when the bacteria are infected with certain
2-15. PATHOGENIC VIRUSES
Table 2-4 presents a list of the principal viral diseases of public health
importance, grouped according to the physiological manifestations, the type of tissue
attacked, or the similarity in the infective virus. These groupings are quite arbitrary, in
that not all authorities classify these diseases in the same way.