How can such a small amount of certain chemicals (e.g. nerve agents, spider venom) be so lethal?

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I’ve seen posts about nerve agents on here before, but this question is NOT about the specific mechanism of how a given substance kills. I’m aware there are many kinds of toxins with varying mechanisms and effects, from weaponised nerve agents, to nicotine, OTC drugs, snake/spider venoms and botulism. And there’s quite a bit of info on those mechanisms readily available online and in previous posts on this sub.

Rather what I don’t understand is how such small amounts of non-reproducing substances can be so deadly? Some of these chemicals seem to be lethal to humans in doses of nano/micrograms. Would it not get diluted by the relative number of molecules in the human body to the point it’s basically homeopathic? And how does such a small dose seemingly so effectively make its way to the specific sites in the body where it causes damage and not just spread out and get lost? There’s only so many molecules in a nanogram of botulinum toxin, how can that possibly be enough to disrupt enough nerves to have the effect it does?

With bacteria and viruses, I understand they can reproduce and actively attack certain cells, so there’s some logical sense to how they are dangerous in small initial quantities, but that doesn’t seem like it’s the case for a chemical. Is it some sort of chain reaction, or something else? Thanks!

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Just because a molecule doesn’t reproduce doesn’t mean it cant interact more than a single time. A lot of the super-toxic chemicals will interfere with other molecules and change how they function. The original toxic molecule isn’t destroyed or locked up in the reaction, so it can continue on wrecking havoc.