I recall reading somewhere that shaving brushes from horsehair were used - and there was belief (likelihood? certainty?) that disease was being spread through the brushes. Thus no more horsehair brushes & the assurance on every brush that it has been sterilized.
of course, I might have dreamed the entire thing....
Horsehair shave brushes were relatively common prior to WWI. Apparently they fell into disfavor as they were a transmission vector for cutaneous anthrax. It was about that time that brushes started to be marked as 'sterilized'.