Four Thieves vinegar was supposedly used by four grave robbers to protect them from bubonic plague, each thief added their own herb to the infusion. It apparently worked well enough, they negotiated their freedom by giving up the recipe.

Nowadays, people vary the herbs, garlic is the constant.

It's no secret herbs like oregano (most savory herbs actually) have antimicrobial properties. When you're poor and a doctor's visit is a day or more lost pay, the daycare is paid regardless of attendance, then the uninsured cost of the visit and pharmaceuticals, you learn.