Quarter-evil in cattle is due to the introduction of a specific bacterium
into the subcutaneous tissue of young cattle. Recovery from the lesions
thus produced is succeeded by immunity. This immunity can be brought
about in various ways; one of them is the introduction of the virus in
small quantities, and here the observation is made that the immunity
stands in direct relation to the quantity of virus injected. Thus, if the
dose of virus in increased, the immunity previously obtained does not
protect, and breakdowns occur (Kitt). This fact is generally observed in
diseases caused by bacteria; an immunity obtained through the introduc-
tion of the minimum lethal doses will protect against this dose, or perhaps
a small multiple of it, but not so against a larger one. ,


Immunity To DIFFERENT VIRULENCY.
Another contingency which occurs in experiments with bacteria is
the different virulency of one and the same bacterium. Artificial
immunity against anthrax, for instance, is obtained by a double
injection of a first and second vaccine, differing in virulency. This
differentiation in virulency can be so exalted that the immunity thus
obtained can be broken by such a virus. This exaltation can be obtained
by passing the virus through a series of susceptible animals of the same
or of different species. It is noted and generally accepted that the more
virulent the virus, the more solid becomes the resulting immunity.
Toe Immune ANIMAL AS A RESERVOIR OF VIRUS.
An important fact to be considered in connection with the immunity
obtained, either naturally or by inoculation in the above-mentioned
diseases, with the exception of pleuro-pneumonia, is the observation that
these animals no longer propagate the disease. The principle holds also


*Nocard and Leclainche, ‘‘ Les Maladies Microbiennes les Animaux,? 1903.
