Two days after twin bombs hit the Boston Marathon, leaving three dead and 183 others hospitalized, two suspicious-looking letters, one on its way to the Capitol Hill office of Sen. Roger Wicker and another addressed to President Obama, have been been intercepted and tested positive for the poison ricin.

Authorities say they see no connection between the suspicious letters and the Boston attacks. Still, the back-to-back events are unsettlingly reminiscent of the 9/11 attacks and the weeks that followed, during which several members of Congress and the news media received letters laced with the deadly bacteria anthrax.

As with the anthrax attacks, the disturbing news of these letters is delivered along with both fear and confusion. What is ricin? Is it deadly? Let’s attempt to answer some of these questions.

1. It Comes From Castor Beans

According to the Centers for Disease Control and Prevention, ricin is a poison that is produced naturally in castor beans—which aren’t actually beans but the seeds of the castor oil plant.

2. It Can Be Quite Deadly

An adult would have to eat about eight castor beans—after first removing the indigestible skin—to die. But only two millionths of an ounce of pure ricin, or about the same weight as one grain of salt, can be fatal. In fact, ricin is 6,000 times more poisonous than cyanide.

Though ricin poisoning is almost always deliberate, it is not likely to serve as a successful means of mass murder. Exposure to the poison is achieved by swallowing it, injecting it, or inhaling ricin that has been sprayed into the air with an aerosol can; so killing a large number of people would probably require adding the substance to a widely distributed food or beverage.

3. The Symptoms Are Slow to Appear

Breaking Bad fans may recall ricin as Walter White’s preferred killing method because it is slow acting—a lethal dose of ricin can take between 36 and 72 hours to kill its victim.

It could be anywhere from four to 24 hours after being exposed to ricin before any of the initial symptoms occur. The symptoms vary depending on how one is exposed. Inhalation can cause nausea, fever, and difficulty breathing, excess fluid in the lungs and, ultimately, low blood pressure and respiratory failure that lead to death. Immediate symptoms of swallowing ricin, on the other hand, include vomiting and diarrhea, which can both cause severe dehydration and low blood pressure. Someone who has ingested ricin may also experience seizures and bloody urine. Within several days, they could die from liver, spleen or kidney failure.

4. There’s No Antidote

But the symptoms can be treated. Depending on how a victim was exposed, they could receive breathing assistance, intravenous fluids to combat dehydration, or anti-seizure or blood pressure medication. If a person becomes aware that they have ingested ricin quickly enough, they could try to flush their stomachs with activated charcoal, a form of processed carbon that is used to clean chemicals spills, purify drinking water, and treat poisonings and overdoses.

5. Ricin Poisoning Is Pretty Rare

Ricin scares are much more prevalent than ricin deaths. According to the Washington Post, only one known ricin death was listed in a 2010 draft of the Homeland Security Department handbook: Bulgarian dissident Georgi Markov was killed with a ricin pellet that was injected into his skin with a James Bond-style umbrella.