I'm looking for a cheat sheet which lists all of the common diseases and their symptoms.
Here ya go:
Other than that, Agent Splat has great advice. "Common diseases and their symptoms" vary by every specialty in medicine, where they practice, what their patient population epidemiology characteristics are, how old their patients are, what ethnicity their patients are, whether they are primary contact or referral, etc, etc, etc ad nauseum. Your best bet would be to make a chart yourself tailored to whatever area of medicine you are attempting to apply it to. Any physician you are working with will be happy to tell you how "common" something you are seeing is, whether its something you'll see 10 times a week, 10 times a year, or 10 times a career.
As an added note, a good sort of "procedure" to go through when developing a differential are:
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Look at the symptoms/PE results and ask yourself what system is involved. Is it CV, GI, urogenital, musculoskeletal, psych, neuro, etc?
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Try to figure out what underlying process is contributing to the malfunction of the system from #1. Is it infection, neoplasm, autoimmune, trauma, endocrine imbalance, idiopathic, etc?
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Then ask yourself what is the etiology of the process. If infection, what bugs are the most common to that form of infection? If a neoplasm, what cancers induce those sort of characteristics and what is the patient's risk factors? Etc.