IDdx is a decision-support software tool developed to assist in the diagnosis of infectious diseases. Having a software tool is not the same as having an infectious disease specialist with years of education and years of experience diagnosing infectious diseases. IDdx is like an electronic book that you can query. It is not designed to replace the human mind, but to help the professional with lists and intersections of lists.
For example, one can make a list of all infections that cause abdominal pain and another list of all infections caused by animal bites. The intersection of these two lists is all infections that fit both criteria. This is what IDdx is designed to do--first to do the research to make the lists and then to design the software that makes the querying of these lists as easy as possible.
Each finding (signs & symptoms) and epidemiological factor in IDdx requires a lot of research to make these lists as complete and accurate as possible. My method in refining the lists is just plain old iteration, defined by Merriam-Webster as "a procedure in which repetition of a sequence of operations yields results successively closer to a desired result."
I look for the best and most recent sources of information, and my starting point is the lastest edition of Control of Communicable Diseases Manual (CCDM). The latest edition of Principles and Practice of Infectious Diseases (PPID) edited by Mandell et al. became available in September 2009 with full-text online access, and CCDM became available in 2010. The availability of searchable electronic resources makes the job easier to build and continuously improve lists for differential diagnosis.
IDdx is regularly updated based on the latest editions of PPID, CCDM, CDC Travel, and other peer-reviewed scientific publications.