why might it be difficult for medical professionals to diagnose a specific illness
Medical diagnosis 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.
Biomedical model According to the biomedical model, health constitutes the freedom from disease, pain, or defect, making the normal human condition "healthy." The model's focus on the physical processes (for example, pathology, biochemistry and physiology of a disease) does not take into account the role of social factors or individual subjectivity. Unlike the biopsychosocial model, the biomedical model does not consider diagnosis, which affects treatment of the patient, to be the result of a negotiation between doctor and patient.[1]