Biosecurity both from the standpoint of protecting people from diseases the animals might be carrying, and from the standpoint of preventing people or objects (shoes, clothing, equipment) from carrying diseases out to spread to other animals. Nowadays they use an outer layer that can be either discarded, disinfected or sterilized.
Sometimes big farms, particularly swine farms, require you to shower and dress their clothing before entering, then change back when you leave.
Sometimes big farms, particularly swine farms, require you to shower and dress their clothing before entering, then change back when you leave.