Mustaches and beards were common among men in the first decade of the 20th century, but military regulations had them shaved clean -because it made a gas mask fit better, and chemical warfare was a real danger to those the front lines. But there were other dangers, especially when the quality of shaving brushes went down.
Before the war, shaving brushes were usually made with badger hair, the preferred bristles for lathering up. When the conflict interrupted the supply of high-quality badger bristles from Russia, suppliers cut a few corners. They switched to horsehair from Russia, China, and Japan—and skimped on a crucial step: disinfection. Instead of sending the hair to France or Germany to be cleaned and sanitized, they sent the tufts directly to brush manufacturers in the United States.
To find out what happened to some soldiers who used the uncleaned shaving brushes, read the scary story at Atlas Obscura.