So Exactly What is Ricin? And Why Is It So Dangerous?

With the recent discovery of two letters mailed from Shreveport to New York Mayor Michael Bloomberg and the gun control group "Mayors Against Illegal Guns" that were laced with ricin, it left us with a couple of questions.  Just what is ricin?  And why is it so dangerous?  

On the website HowStuffWorks.com we read:

Ricin is a toxin that is fatal to humans in extremely small doses.  Just 1 milligram is a deadly amount if inhaled or ingested, and only 500 micrograms of the substance would kill an adult if it were injected (CDC). Ricin comes from the castor bean plant -- it is present in the mash that is left over after grinding castor beans into oil. It can be delivered as a powder, a mist or a pill.

Additionally, we get this from CNN.com:

If inhaled, injected or ingested, less than a pinpoint of ricin can kill a person within 36 to 48 hours due to the failure of the respiratory and circulatory systems. There is no known cure.

One molecule of ricin will kill a cell it enters.

Ricin ingestion is not always fatal. Children are at more risk than adults.  If ingested, ricin causes nausea, vomiting and internal bleeding of the stomach and intestines, followed by the failure of the liver, spleen and kidneys, and finally death by collapse of the circulatory system.

However, medical experts point out that ricin is nowhere near as powerful a killer as anthrax.