infinitesimal

细菌的体积是如此微小，以至于肉眼几乎无法察觉。

The size of bacteria is ___infinitesimal, almost invisible to the naked eye.

被闪电击中的几率微乎其微。

The odds of being struck by lightning are ___infinitesimal.

样本中的污染物浓度极微，远低于安全阈值。

The concentration of the contaminant in the sample was ___infinitesimal, well below the safety threshold.