Five letters containing anthrax spores were mailed to various media outlets in the United States.

Anthrax is an infection caused by the bacterium Bacillus anthracis or Bacillus cereus biovar anthracis. Infection typically occurs by contact with the skin, inhalation, or intestinal absorption. Symptom onset occurs between one day and more than two months after the infection is contracted. The skin form presents with a small blister with surrounding swelling that often turns into a painless ulcer with a black center. The inhalation form presents with fever, chest pain, and shortness of breath. The intestinal form presents with diarrhea, abdominal pains, nausea, and vomiting.

The 2001 anthrax attacks, also known as Amerithrax, occurred in the United States over the course of several weeks beginning on September 18, 2001, one week after the September 11 attacks. Letters containing anthrax spores were mailed to several news media offices and to senators Tom Daschle and Patrick Leahy, killing five people and infecting 17 others. Capitol Police Officers and staffers working for Senator Russ Feingold were exposed as well. According to the FBI, the ensuing investigation became "one of the largest and most complex in the history of law enforcement".