The Bible has a detailed description of the priestly costume in Exodus 28. Aaron and his priestly descendants certainly looked fabulous, but if the Bible can spend an entire chapter on this, why not a method for making something useful, like soap?
It’s not hard to make. Imagine if the following recipe were a quote from the Bible (give it a King James tone if that makes it sound more authentic):

Pack a wooden bucket with wood ashes. Pour in boiling water. Make a small hole near the bottom so the water can be collected in a pot as it drips out. The liquid is caustic, so don’t let it touch skin or metal. Pour the liquid back through the ashes until it is strong enough to dissolve a chicken feather.
Boil this liquid until most of the water is gone. Add rendered fat from cattle or other animals and stir while cooking until it thickens. Pour into molds and let it harden.

There are lots of tricks to making soap properly, but a priesthood could’ve easily perfected the technique.
With this, the Bible could then add the basics of health care—when and how to use this soap, how water is purified by boiling (actually purified, not just pretend purified with a ritual), how latrines should be built and sited, how to avoid polluting the water supply, how to avoid spreading disease, and so on. Other ideas to improve society come to mind—low-tech ways to pump water, spin fiber, make metal alloys, and so on—but health seems to be a fundamental one to start with.
Several passages have been advanced to argue that the Bible did refer to soap. Malachi 3:2 and Jeremiah 2:22 allude to it, but that word means ashes or soapy plant. In Job 9:30, the word isn’t soap but “snow water” (that is, pure water). Numbers 19:1–12 has been claimed as a recipe for soap, though it’s clearly just a ritual. None of these are soap as we would understand it, as defined by the recipe above. If the Bible did have a recipe for soap, wouldn’t we read in the Bible about people using it and the health improvements that came from the new practice?
The Bible has an abysmal relationship with science (more here and here), unless you see it as simply another book of mythology and superstition, in which case it’s a product of its times like all the rest. Jesus does no better with his attempts at medicine. Wouldn’t someone who preached “Love your neighbor as yourself” bring his A game to the problem of public health?
Another attempt to salvage the Bible argues that its odd dietary rules (no pork or shellfish, no mixing of meat and dairy, etc.) are healthy, but these rules are arbitrary when seen from a modern standpoint. Sure, avoiding pork means that you can’t get sick from eating poorly cooked pork, but can’t you still get sick from eating tainted meat from other animals? An analysis by Mary Douglas (discussed here) makes much more sense out of the ritual prohibitions.
There are two possibilities for the Bible’s health advice.

• An infinitely loving God created us but just didn’t give a hoot about the health of his creation. He could’ve made healthy practices mandatory rituals, but he didn’t. However, he did care enough about making his priests look sharp to devote an entire chapter to their costumes.
• The Old Testament was just written by ordinary men and reflects their ordinary knowledge and interests.

Which seems likelier?

Man once surrendering his reason, 
has no remaining guard against absurdities the most monstrous, 
and like a ship without rudder, is the sport of every wind.
— Thomas Jefferson

(This is an update of a post that originally appeared 3/13/13.)
Image credit: Arlington County, flickr, CC