How can i get a database of all diseases, sicknesses ,ailments etc and all its associated symptoms?
You can’t because it doesn’t exist. There are many databases containing subsets of the data but due to the nature of research and diseases there is not a complete one.
Have a look at Bioportal, which is gives a comprehensive listing of openly available coding systems in the biomedical domain. Coding systems of interest for you - among others - are:
- SNOMED-CT (Standard Nomenclature of Medicine - Clincal Terms)
- MeSH (Medical Subject Headings)
- MedDRA (Medical Dictionary for Regulatory Affairs)
- ICD9, ICD10 and upcoming ICD11
All of these are work in progress (kind-of).
Which of these is best, depends on an analysis of your requirements. I would also recommend to have a look here: http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1004743