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That Blocks Virus: During the course of adaptation to rabbits the virus has increased in virulence for the new host and at the same time lost virulence for the dog and for man. Such fully adapted virus is known as fixed virus. The rabies vaccine of Pasteur consisted of an emulsion of spinal cord of rabbits infected with fixed virus. Later the rabies virus was also adapted to the chick embryo, but living vaccines prepared in this manner are still in an experimental stage. Other rabies vaccines in use consist of killed virus, infected spinal cord treated with carbolic acid (Semple vaccine) or ultraviolet light to inactivate the virus.
It has long been suspected that infectious mononucleosis, predominantly a disease of young adults, is caused by a virus; but attempts to isolate the virus have been unsuccessful. Recent evidence, however, indicated strongly that the agent responsible is the virus of Burkitt's lymphoma, a virus that is associated with a rare tumor of man in certain regions of Africa. It now appears that a high proportion of adults in other parts of the world have developed antibodies to the virus, indicating previous infection with this or a closely related virus; a number of patients with infectious mononucleosis exhibited an acute rise in the level of these antibodies in the course of the infection.
As with the synthesis of an enzyme, it has been known for some time that the sudden multiplication of lysogenic viruses can also be induced. This occurs when bacteria that harbor the viral chromosome in a latent state are exposed to certain chemicals or to physical agents such as ultraviolet light or X rays. Then the virus breaks off from the chromosome and begins to replicate. The mechanism that maintains the viral chromosome in the dormant state was proposed (by the French scientists) to be the same as that which inhibits the function of normal cellular genes—that is, the attachment to the viral DNA of a specific protein represser that blocks virus multiplication. |
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