Poisons, bacteria, venoms, viruses, parasites, drugs, mutations.
Anything that: kills you, affects your behaviour, kills you under specific conditions, makes you turn into a tree or makes you produce alcohol in your system.

I love those tiny exotic frogs who can touch anyone and they kill him. But the frogs don't die from touching each other.

And Black Widow venom doesn't affect cats' nervous system.

And frogs can stay alive even if their guts are hanging out because their cells don't coagulate.