Soap, then germ theory and antisepsis, then antibiotics and vaccines.
Make the cheapest life-saving technology ever invented.
Needs first: Fire, and keeping it (Energy), Livestock (Food), Containers (Materials)
Establish that disease is caused by organisms — and act on it.
Needs first: Glass (Materials), Plant medicine and the pharmacy, Soap
Make infection survivable.
Needs first: Germ theory and antisepsis, The generator (Energy), Electrochemistry and light metals (Materials), Instruments and measurement (Knowledge), Refrigeration and the cold chain (Food)
Every guide, build, reference sheet and book reached from the steps above.
The two dozen that decide who lives where there is no doctor: what each does, how it spreads, what kills it and which step stops it.
Level 0The distance between a latrine and a well, a bar of soap, and a covered pit prevent more deaths than every hospital on the ladder combined.
Level 0Without antibiotics, a cut is a slow emergency and a deep bite is a fast one. What saves people is mostly pressure, water, and knowing which wounds must be left open.
Level 1The poisonings a rebuilding community is most likely to meet: where each comes from, how it shows, and the first thing to do.
Level 3What matter is made of and how it changes: amounts, gases, acids, the energy and speed of reactions, electrochemistry, and the reactions every industry on the ladder runs on.
Level 3Fourteen branches, each with the handful of results a civilization cannot rebuild without, and where to learn the rest.
Level 3The basic set from the WHO list: what each treats, what it is made from, and the earliest tier that can make it. No doses, on purpose.
Level 3Life from the cell up: the human body, microbes and immunity, inheritance and epidemics. Each result with what it lets you do and who first found it.
Break the faecal–oral route and return the nutrients — the highest lives-saved-per-effort structure a settlement can build.
A glass bead the size of a pinhead magnifies 150–350 times: enough to see yeast, blood cells and the larger bacteria that cause disease.