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(power of) resistance
睡眠不足は抵抗力を弱める。
Lack of sleep weakens resistance.
栄養をしっかり取れば抵抗力がつく。
You build up resistance by eating well.
冬場は抵抗力が落ちやすいので注意したい。
You should watch out because resistance tends to drop in winter.