You should know about coughs and colds, measles and other childhood illnesses. It’s probably good to know about things you’ll see in others, particularly old folk, like arthritis, heart conditions and cancer. And it wouldn’t do any harm knowing about the likes of cirrhosis of the liver. But all this is really only knowing what these things are, you don’t need a doctor’s knowledge. It’s the same as with a car – you don’t have to be a mechanic to drive it sensibly.
Isn’t the internet wonderful? This is part of the website of a doctor’s surgery in Gerrard’s Cross - http://www.calcotmedicalcentre.co.uk/common_illnesses.htm - it gives pretty much what I had in mind for this section. I doubt whoever wrote it thought he might have a much wider audience!
And this one from PAGB, the UK trade association for manufacturers of branded over-the-counter (OTC) medicines and food supplements, is particularly helpful http://www.chic.org.uk/chicmco/feelingunwell/index.htm