GENERAL PATHOLOGY. 
159 
of the disease among experimental fowls for nearly eight months, 
and though his home flock is but a short distance from them, 
. but a few of these have sickened, and then the disease has been 
checked with the loss of a single bird in each instance. It is be¬ 
lieved that the birds which thus contracted the disease were 
infected by flies, which would gorge themselves with virulent 
blood in the laboratory, where dissections were made, and then 
fall victims to the poultry which were running about outside. 
No cases have occurred in this manner since the cold weather has 
destroyed these insects. 
The experiments on which the above regulations are founded 
will be detailed in future reports of this Department; they are 
sufficiently numerous to be worthy of the fullest confidence. 
The value of the method of preventive inoculation or vaccina¬ 
tion discovered by Pasteur has not yet been decided, but in view 
of the comparative ease with which the affection may be controlled 
by the measures detailed above we doubt if it can ever be advan¬ 
tageously adopted as a means of preventing this particular 
disease. 
D. E. Salmon, D.Y.M. 
Ashville, N. C., Feb. 18, 1881. 
GENERAL PATHOLOGY. 
THE ATTENUATION OF VIRUSES AND THEIR RETURN TO VIRU- 
LENCY. 
By M. L. Pasteor. 
In recent publications, I have published the first example of attenuation of a 
virus by the only resources of experiment. Formed by a special microb extremely 
small, this virus may be multiplied by artificial cultivations outside of the bodies 
of animals. These cultivations, left without possible contamination of their con¬ 
tents, with him, undergo more or less deep alterations in their virulency. The 
oxygen of the air presented itself as the principal cause of these changes, viz., of 
these diminutions in the power of multiplicity of the microb; for it is evident 
that the virulency is mixed up in its different powers with the different faculties 
ofgrowth of the parasite in the economy. 
It is not necessary to insist upon the interest of those results, nor on their 
deductions. To look to diminish the virulency by ordinary means, is to base on 
experiment the hope to prepare with active viruses, of active cultivation in the 
