Ricin

Taken from http://www.chem.ox.ac.uk/mom/ricin/ricin.html

Ricin, derived from the castor bean is a potent toxin which can kill a human with small doses.  It acts by inhibiting protein synthesis.  Symptoms of ricin poisoning can include bleeding, vomiting, diarhea, coughing and difficulty of breathing.

Fortunately, although the castor bean is common, ricin does not occur naturally, and so it would take a deliberate act to be exposed to ricin.  there is no known cure for ricin; however, it has medical potential in bone marrow transplants and as a treatment for cancer, by killing the cancerous cells.

Ricin has suspected of being used in the Iran - Iraq war.  It has also been used by terrorists.  In February 2003, three men appeared in court on charges of conspiring to produce ricin.