= Haemophilus ducreyi
{c}
<Haemophilus ducreyi> is a <bacterium> associated with <chancroid>, an <infection> producing painful genital ulcers. The organism and the clinical condition are different concepts: a similar ulcer can have another cause, and laboratory evidence helps distinguish those possibilities. <Chancroid treatment guidance by CDC>[Diagnostic Considerations: H. ducreyi and the clinical pattern; no treatment regimen reproduced]{ref}
This gives a concrete example of the difference between a symptom and an infectious agent. A lesion is an observable <tissue> outcome, whereas identifying the agent concerns its cause. Clinical appearance, detection of bacterial material and exclusion of competing explanations answer complementary questions. A negative result from an imperfectly sensitive test also does not logically establish absence of the organism.
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