It is dangerous to keep bad company even if that is only one person because
that _rotten_ one can have a very bad influence on the rest for the friends.
The proverb teaches prudence in keeping company with people that have vices
since those can be contagious and catch the rest of the group the same way
that bacteria from one rotten apple catches the other apples near it.

##### See also:

* He should have a long spoon that sups with the devil
* He that toucheth pitch shall be defiled
* Who keeps company with the wolf will learn to howl