Unknown to many these days, it used to be common to have your shaving brush made out of horsehair. Horsehair for a brush makes sense on many levels, not the least of which is the fact that they are animal friendly - the hair is harvested as part of the normal grooming of the horse.
However, the horsehair brush fell out of favour around the Great War, since some suppliers were less than good about sterilizing their products. A bit of searching online brings to light some documents from the time:
Anthrax from Shaving Brushes (July 12, 1918)
Anthrax and the Sterilization of Shaving Brushes (May 9, 1919)
An Investigation of the Shaving-Brush Industry, with Special Reference to Anthrax (May 9, 1919)
Isolation of Anthrax Bacillus from a shaving Mug (November 1, 1922)