Some varients of botulinum poison are so deadly that even a few nanograms can kill a person. How does a few nanograms of anything kill someone?
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Botulinum poison targets proteins your body uses to make cholinergic neurons release acetylcholine. It binds to them and prevents them from doing their job, which is critical to normal operation of billions upon billions upon billions of very important neurons in your body. However, if you were to estimate the total mass of all of these very important and specific proteins in your body, you would probably get an answer in the… drum roll please… nanogram range. There is simply not a lot of the target compared to the sheer volume of Other Stuff hanging around.
Another important thing to consider is that the bacteria that produce botulinum toxin have had billions upon billions of years to evolve small changes to the genetic code for the toxin, and there is some logical evolutionary pressure on forms of the toxin that are better at binding to the target (and poorer at binding to the Other Stuff). It only takes nanograms to kill in part because the botulinum toxin is so precisely refined towards performing its functions and ONLY its functions, which are binding to and inactivating stuff it would like to inactivate. This is also the reason why levels of common steroid hormones like testosterone and estrogens are universally in the nanogram range — that’s all you need when billions of years of evolution have ensured that this square peg can only, ONLY, fall into the square hole.
(Also, the steroid hormones are poorly soluble in water, so you can really only get nanogram concentrations into the blood/cytosol anyways, but that’s outside the scope of this discussion and not totally relevant.)
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