In a harmful relationship, you may feel the friend is insulting, critical, petty or selfish.

在一段有害的朋友关系中，你可能会觉得这位朋友侮辱人、爱挑剔、小气或自私。

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Knowing how valuable friendship is, we should be very careful in making friends. A good friend is helpful to us while a bad friend would be harmful.

知道友谊的珍贵，我们就该谨慎交友。好朋友对我们帮助很多，而不好的朋友则贻害无穷。日久见人心。

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Over time, the babies' immune systems start to distinguish between friend and foe in the microbial world, and launch attacks on potentially harmful bugs while leaving beneficial ones, like those that live in the gut, alone.

CNN: Connection between dirty diapers, childhood health