_Low condition_ or a _low tone_ of health from any cause predisposes. Old, worn out animals, ewes in lamb, or those just lambed, sheep that have been shut up and denied proper exercise in winter, those on poor feeding and perhaps nursing twins, those that have suffered from any debilitating disease of any kind are especially obnoxious to a dangerous attack. _Microbiology._ The microbe, which Lignieres found in the pulmonary lesions, is one of the colon group of pathogenic bacteria that have been classed together as _pasteurella_. It usually appears as a very minute ovoid bacillus which stains promptly and deeply at the poles in fuchsine or gentian violet, leaving a clear median part, so that it seems a _diplococcus_. It bleaches readily in Gram’s solution. Its form varies in different culture media sometimes showing long _bacilli_, and sometimes _streptococco-bacilli_, but the usual and characteristic appearance is that of a _cocco-bacillus_, and to this it constantly returns. The microbe is ærobic and nonmotile (the slow zig-zag motion sometimes seen does not seem to be automatic). In peptonised bouillon it produces opacity in 18 hours, or in simple bouillon in 24 to 48 hours, the best temperature being 100° F. Gelatine plate cultures are slow because of the compulsory low temperature, yet in 36 to 48 hours it forms pale blue, translucent, round colonies the size of a pin head. It never liquefies. In coagulated blood serum it forms only a thin transparent pellicle hardly visible, and there is no growth on potato.