<P> Medical diagnosis (abbreviated Dx or D) is the process of determining which disease or condition explains a person's symptoms and signs . It is most often referred to as diagnosis with the medical context being implicit . The information required for diagnosis is typically collected from a history and physical examination of the person seeking medical care . Often, one or more diagnostic procedures, such as diagnostic tests, are also done during the process . Sometimes posthumous diagnosis is considered a kind of medical diagnosis . </P> <P> Diagnosis is often challenging, because many signs and symptoms are nonspecific . For example, redness of the skin (erythema), by itself, is a sign of many disorders and thus does not tell the healthcare professional what is wrong . Thus differential diagnosis, in which several possible explanations are compared and contrasted, must be performed . This involves the correlation of various pieces of information followed by the recognition and differentiation of patterns . Occasionally the process is made easy by a sign or symptom (or a group of several) that is pathognomonic . </P> <P> Diagnosis is a major component of the procedure of a doctor's visit . From the point of view of statistics, the diagnostic procedure involves classification tests . </P>

Why might it be difficult for medical professionals to diagnose a specific illness
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