Ricin

Ricin is a poison. It is a highly toxic, naturally occurring protein. It is produced in the seeds of the castor oil plant "Ricinus communis". 
A dose the size of a few grains of table salt can kill an adult human. The median lethal dose (LD50) of ricin is around 22 micrograms per kilogram (1.78 mg for an average adult, around of a standard aspirin tablet/0.4 g gross) in humans if injected or inhaled. Oral exposure to ricin is far less toxic and a lethal dose can be up to 30–40 milligrams per kilogram.
Ricin prevents cells from assembling various amino acids into proteins, and death occurs after a few hours up to a day. Ricin has been used as a terrorist weapon, including the assassination of Georgi Markov in 1978, supposedly by the KGB.