Lacking a clear cause or origin, especially when describing a disease whose cause is unknown or cannot be identified.
From Greek 'an-' (without) + 'aitia' (cause) + '-logical' (relating to study), used in medicine when doctors can't figure out what's causing an illness.
Many diseases were called 'anaetiological' until modern science caught up—fibromyalgia was mysterious for decades until researchers realized it involved nervous system sensitivity, showing how this term reflects the limits of medical knowledge.
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