I remember reading decades ago (back when the population of the earth was “only” 2 billion) that a water-glass full of pure botulism toxin, properly distributed, could kill the entirety of humanity.
A dose as small as a few grains of salt can kill an adult.
The LD50 (Leathal dose for 50% of those exposed) of ricin is around 22 micrograms per kilogram (1.76 mg for an average adult, around 1⁄228 of a standard aspirin tablet/0.4 g gross) in humans if exposure is from injection or inhalation.
Ricin has been involved in a number of incidents, including the high-profile assassination of Georgi Markov using a weapon disguised as an umbrella.