bad influence

dad joke

is a threat to the public good --- one bad apple will spoil the whole barrel.

One bad apple will spoil the whole barrel.

is a real threat --- The rotten apple spoils his companion.

is dangerous --- A wicked companion invites us all to hell

A wicked companion invites us all to hell

is dangerous --- you ought not keep ill men company, lest you increase the number.

Keep not ill men company, lest you increase the number.

is best avoided --- Heaps of good cotton stalks get chopped up from association with the weeds.

Heaps of good cotton stalks get chopped up from association with the weeds.