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Try to understand what the tool (IDdx) can do and cannot do.
A. Symptoms are included only if they are present in any of the 12 main references.
B. For example, some references may list pneumonia as a rare complication of disease X without mentioning cough or infiltrate. In this instance, IDdx will not show cough or infiltrate as symptoms of disease X, but only pneumonia as a complication.
Diseases covered are similar to those in CCDMplus 37 "Localized Infections."
A. Some diseases are covered as a group, for example, "Arthropod-borne viral arthritis and rash" and "Viral encephalitides, mosquito-borne." Specific diseases within the group are listed in the synonyms and diseases index. 

B. Not covered in IDdx are congenital and neonatal infections.

*CCDM = Control of Communicable Diseases Manual

The aim is to list all common findings for each infectious disease (except for findings of congenital and neonatal infections).
A. The general symptoms (prefixed with the > symbol) are usually present as initial symptoms.
B. The complications (prefixed with the * symbol) may be extremely rare for the disease, but they are still important to describe because of their impact on individual and public health. For example, only about 1 per 100,000 patients with measles develop subacute sclerosing panencephalitis.

When building a differential diagnosis list, think "zoom-intersection" rather than a laundry list of every symptom.

A. General symptoms like fever and fatigue are not very useful in narrowing down the list.
B. An effective query will combine one or more findings with one or more epidemiological factors.
C. The opening search screen also provides five additional ways to narrow the list: Category (including "Exclude Localized Infections"), Endemic, Acuity, Incubation Period, Occupation, Immunocompromised at Risk, and Only Bioterrorism Diseases.

You may limit the results by selecting a category of Acuity.
A. Be aware that these categories overlap, and that diseases can present in different ways.
B. For example, while tuberculosis is classified as subacute/chronic, it may sometimes present as either acute-severe (emergency admission), or acute-moderate (recent onset of symptoms treated in a clinic).

C. For some queries, you may get better results using one or more incubation periods.
The default search is an AND query. You may find some uses for an OR query (Match Any).
A. For Endemic, select all regions that the travelor visited.
B. For Incubation Period, select all options that may apply in the particular case.
C. For Occupation, if you want to select a category instead of a single job, then check all jobs in the category.
The purpose of the Cases/Year fields is to enable sorting of diseases by most common to least common.
A. The numbers are usually incidence rates, but for some chronic diseases like strongyloidiasis, are based on prevalence figures. 
B. Round off numbers of 300 million and 6 billion are used to represent U.S. and global populations. (20 X 300 million =  6 billion.)
Print these PDF documents to assist in building queries: