Chemicals active at the nanogram scale are invisible, difficult to observe, and near-impossible to clean or remove; but they are still psychoactive enough to torment you like demons.
Chemicals at the nano-scale, which are invisible to ordinary thinking, can still have large effects at the human-scale: examples, despite all precautions, include the bizarre phenomenon of “disappearing polymorphs” (crystals which can ‘infect’ other crystals, causing severe harm to drug production) and highly potent drugs which cause overdoses.
Such mysterious behavior could be abused. I dramatize this by speculating on how the spicy chemical resiniferatoxin (RTX) could be employed as a nanotech tool of psychological warfare due to its sub-microgram dosage—it would be an ineradicable, invisible, inexplicable curse on the victim, who would suffer continuously in confusion, almost supernaturally.
But it would be completely natural: merely a very small bit of Nature.