So historically, lice and other parasites were a major vector for diseases like typhus. Particularly when a group of people were forced to sleep in close proximity, such as soldiers in barracks, refugees in tents, a community in a cold climate that would cram in to a single winter shelter, or travelers in a bunk house.

Shaving hair in general was an important method for reducing transmission, as it reduced the space they could hide in. Regular washing of the body. Clean beds with frequently replaced fillers (like straw) and bed linings laundered regularly and between separate users.

One reason many early modern soldiers are depicted as clean shaven was because it became a mandatory practice in many armies to minimize spread of disease. As regular washing, separated sleeping quarters, and regularly washed bedding was not practical for such a large group on the move. Even now, many militaries will shave new recruits to the scalp, to make sure they don’t bring any hair borne parasites in to the barracks. (Later on face shaving became extra important as a way to ensure gas masks would seal properly)

Lots of ways this was mitigated. It’s fairly rare these days due to most people regularly washing, getting parasites like bed bugs or lice exterminated the moment they’re found, and beds that are much less suited to harboring them.

But a lot of the hang ups around body hair are cultural hangers on from a time when this was a much more significant issue. In all likelihood it would become an issue again if society as a whole slackened on many of these practices at once.