The Definitve S&S Handbook....the Holy Grail...
I understand, through no definitive sources, that NASA tried to develop diagnostic algorithms, and failed. What happens is that so many maladies can present in so many ways, and so many maladies present along with others, that there is an invisibly thin tipping point between the thing petering out in zebras (too many answers) or abruptly crashing by having too few possibilities. Also, since you need to be balancing pt history and description of symptoms against your observations, it would be too cumbersome to try to describe the winnowing process your mind makes in diagnosing. Finally, while you can break down all decision making into yes/no decision trees, the process to define say a wart or a second degree burn would take pages and pages. Imagine differentiations like between a common cold and CHF?
Curiosity into that can be the first step into getting a higher certification or a degree. Go get your Merck and your A&P text and put em by the bed or on the back of the toilet, and remember to follow your protocols anyway until you write or graduate to new ones.
That's why it is an art backed by science.