On Tuesday afternoon, news broke that an envelope addressed to Sen. Roger Wicker (R-Miss.) had tested positive for a deadly poison known as ricin. On Wednesday morning, the Secret Service revealed that a letter addressed to President Obama may have been laced with the same toxic substance. What is it, exactly? Here's what you should know:

What is ricin?
A slow-acting poisonous chemical found naturally in castor beans. As little as 500 micrograms of it — about the size of the head of a pin — can kill an adult. According to the Centers for Disease Control and Prevention, ricin is made from the waste left over once the beans are processed into castor oil (used typically as a laxative or auto lubricant), and can come in powder, mist, or pellet form. In medicine, it's been experimentally used to kill cancer cells, and the U.S. military even experimented with weaponizing the agent on a large scale in the 1940s.

How does it hurt people?
For ricin to do harm, it has to either be ingested, inhaled, or deliberately injected into the bloodstream — it can't be spread person-to-person. In a famous 1978 incident, Bulgarian dissident writer Georgi Markov was murdered by an assassin wielding an umbrella dart gun. His poison of choice? Ricin. 

What does it do to the body?
Ricin burrows into the the body's cells and effectively prevents them from producing new proteins. Without new proteins, the body's cells die. Without healthy cells, you die, too. Its slow-acting nature is why it can take hours for symptoms of ricin poisoning to become evident. According to the CDC, symptoms typically manifest within 4 to 8 hours, but can occur as late as 24 hours after the ricin enters the bloodstream. Death can take place 36 to 72 hours after exposure. 

What are the symptoms of ricin poisoning?
Things can get pretty gruesome and nasty. If inhaled, a person might have difficulty breathing, develop a fever, cough, nausea, and may begin turning blue. If ingested, the ricin can cause a person to discharge bloody vomit or diarrhea. The poison has also been known to trigger seizures and to shut down vital organs.

Is there an antidote?
No. The only way to counteract the toxin is to remove as much of it from the body as possible.

How likely is it that a letter laced with ricin would kill its intended target?
Not very. Letters to elected officials like the president and U.S. senators are screened at off-site facilities before being delivered. Unfortunately, though, that means someone lower on the totem pole is probably at risk if they are handling letters addressed to high-ranking politicians.