HOUSTON -

The nation is breathing a collective sigh of relief after investigators arrested a suspect for allegedly mailing letters with suspected ricin poison to President Obama and other elected officials.

Authorities took 45-year-old Paul Kevin Curtis into custody at his home in Corinth, Mississippi.

Discovered just a day after the Boston Marathon bombings, the ricin letters raised fears from Arizona and Texas, all the way to Capitol Hill.

Ricin isn't just harmful. Even a tiny amount can quickly and easily become deadly.

"If it gets into your body it can be highly toxic," said Dr. Anthony Maresso, a professor of virology and microbiology at Baylor College of Medicine.

Maresso said the toxin is processed from the castor bean plant, which is grown in several states, including Texas. The ricin is usually ingested by breathing and symptoms can kick in within two-to-three hours.

"If left untreated, patients can succumb to intoxication anytime after that," said Maresso. "Most typically between two and five days."

The FBI doesn't believe the ricin letters are connected to the Boston bombings, but both cases remind Americans we can still be vulnerable.