A dose of poison can do its work but once. A bad book can go on poisoning minds for generations.

The dose makes the poison.

Depending on the dose, it can be immediately toxic. It is a (poison).

I mean, I have the feeling that something in my mind is poisoning everything else.

Malice drinketh its own poison

Goats wouldn't have seen a tiger from an evolutionary point of view for at least 15 generations but they recognize the smell of the predator.