I was bored and started thinking what is the deadliest substance/thing that could kill a human adult that is also either very tiny. I came up with ricin, as I understand only a few grams of the stuff could be deadly, although I might be wrong. I think that was the toxin used in the spy killings when a spy stabbed someone with an umbrella tip that injected a tiny ball full of ricin.
Okay, so you get my drift. What is the smallest amount of something that can kill an adult?
Seriously, though, the amount is dependent upon the time the assailant is willing to wait for the victim to succumb.
There are bioweapons with very low dosages that will cause death in 5 days or so.
Botulinum toxin (of which Botox is one variety) might be the winner. According to Wiki:
“When introduced intravenously in monkeys, type A (Botox Cosmetic) of the toxin exhibits an LD50 of 40–56 ng, type C1 around 32 ng, type D 3200 ng, and type E 88 ng[citation needed]; these are some of the most potent neurotoxins known.”
According to the list here, botulinum toxin is about 10 times more toxic than polonium, and 20,000 times more toxic than ricin, but only if the latter is injected; the oral dose is a thousand times greater still, with ionizing radiation the only thing that exceeds it (what would the weight of 3-6 Gy of radiation be? Very small in any case, especially gamma rays).
How much damage would one microgram of antimatter cause? Would it crater a fist sized hunk of flesh? Vaporize a pile of anvils? Make newspapers smolder?
There are supposedly some fentynal analogues that are so insanely potent even one single grain or speck touching your skin could be fatal. For a chemist to work with it requires full bunny suit with air filter, and keep a syringe full of nalaxone handy in case your suit gets breached. Thats pretty crazy.
Humans. One of us can do a lot of damage to a large group of people while theses toxins generaly affect a single individual. At least in the quantaties above.