Castor Oil Plant seeds, both fresh green and dried, showing entire pods, and the pod split into three parts, and the exposed seed. 
What it is:
How much is too much: It takes the pulp of about eight beans to kill an adult. If the protein is purified from the beans, a very small amount — less than 2 milligrams if injected — will kill a person. If eaten, about 2000 milligrams will kill someone.
Where it comes from: The waste from castor oil manufacturing, called waste mash, contains about 5% to 10% ricin by weight, but if it has been heated (which it usually is during the manufacturing process) the protein is no longer toxic.
To capture toxic ricin from castor beans, the protein needs to be separated out from the other plant proteins. There's a patent that details how to do that, though it might not be all that effective. It probably takes a pretty good chemist to isolate the ricin protein in pure form that is still active.
If the ricin is going to be used as an inhalable poison, it needs to be adapted to a very small particle size to be effective, though it could also be dissolved in water and inhaled as a mist.
How it works: Ricin can kill you if it is inhaled, injected, or ingested. It is most potent when inhaled or injected into the body. Bulgarian writer Geogi Markov was murdered by the Bulgarian secret police by an injection of ricin disguised in an umbrella.
Ricin stops your cells from making proteins by attacking our protein-making factories, the ribosome. Proteins are an essential part of every part of your body.
How long does it take: Ricin isn't the quickest-acting toxin. It can take up to a day after exposure for symptoms to show up, but it can kill you within three days. If you survive more than five days, you are likely to live.
What are the symptoms: Victims who ingest the protein get severe nausea and diarrhea. If it is inhaled, the patient will also have difficulty breathing and fluid will build up in the lungs.
They end up with an irregular heartbeat, low blood pressure, and seizures. This can last for up to a week. The patient can die of shock and multiple organ failure.
If you ingest the protein, treatment can usually reverse the symptoms and patients will survive, though they may suffer long-term organ damage. A man was poisoned with ricin in Las Vegas in February of 2008 and fell in to a coma for more than a month.
The Bulgarian writer Markov first showed a fever, was admitted to the hospital, and died three days later.
How is it treated: Treatment basically tries to minimize the effects of the symptoms of the poison. This could include ventilation to help breathing, giving fluids to help stave off dehydration, and giving medication to stop seizures and raise blood pressure.
The U.S and U.K. militaries have developed an antidote to ricin, but they haven't been fully tested on humans. Researchers have also been trying to develop a vaccine for it.
Ricin is not all bad. Some researchers have been trying to use the toxin to kill cancer cells in the body. That is possible because the protein can be linked to a cancer-cell-specific antibody, which can go through the body without hurting normal cells.