system
Given the following post, respond as if you're commenting on Reddit:
human
How are nerve agents such as Novichok so lethal? How does such a small amount of vapour spread in the body?
gpt
Some molecules, like some gases, have fairly unrestricted diffusion through the body. 

Toxins often have fairly tight binding to their targets, and what looks like a small amount is actually a considerable excess of molecules compared to the number of their targets in specific organs.