In 1969 -- when Bulgaria was still part of the Soviet Bloc -- a political dissident named Georgi Markov defected from Bulgaria, eventually settling in London, where he campaigned vigorously against Soviet oppression.
While waiting for a bus on a London street in 1978, Markov was poked in the leg by the tip of someone's umbrella. The umbrella, it turns out, stung Markov with a tiny amount of ricin. He died shortly thereafter, and his murderer (suspected of being a Soviet agent) was never found.
Ricin kills by inhibiting protein synthesis in cells, according to the CDC. Symptoms of ricin poisoning, which can take several hours to show up, vary depending on whether the toxin was inhaled, ingested or -- as in the case of Markov -- injected.
Symptoms may include fever, difficulty breathing, fluid in the lungs, vomiting, diarrhea, dehydration and blood in the urine. Death can result within 72 hours; even survivors may have long-term organ failure and other health problems.
There is no known antidote for ricin poisoning.