The great physicist Richard Feynman once posed a similar question: “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.”
That certainly does encapsulate a huge amount of understanding, but it also wouldn’t be particularly useful, in a practical sense. So, allowing myself to be a little more expansive than a single sentence, I have some suggestions for what someone scrabbling around the ruins of civilization would need to know about basic necessities.
You would need to start with germ theory — the notion that contagious diseases are not caused by whimsical gods but by invisibly small organisms invading your body. Drinking water can be disinfected with diluted household bleach or even swimming pool chlorine. Soap for washing hands can be made from any animal fat or plant oil stirred with lye, which is soda from the ashes of burned seaweed combined with quicklime from roasted chalk or limestone. When settling down, ensure that your excrement isn’t allowed to contaminate your water source — this may sound obvious, but wasn’t understood even as late as the mid-19th century.