• 熱ねつ — fever; heat
• 熱心ねっしん — enthusiastic; devoted
• 熱意ねつい — zeal; enthusiasm
• 高熱こうねつ — high fever
• 発熱はつねつ — generate heat; fever

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熱 combines the fire radical 灬 (flames) with 埶 (phonetic element). It expresses physical heat and fever, but also strong feelings like zeal and passion. It is widely used in both medical and emotional contexts.