First mailing of anthrax letters from Trenton, New Jersey in the 2001 anthrax attacks.

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.

Trenton is the capital city of the U.S. state of New Jersey and the county seat of Mercer County. It was the capital of the United States from November 1 until December 24, 1784. Trenton and Princeton are the two principal cities of the Trenton–Princeton metropolitan statistical area, which encompasses those cities and all of Mercer County for statistical purposes and constitutes part of the New York combined statistical area by the U.S. Census Bureau. However, Trenton directly borders the Philadelphia metropolitan area to its west, and the city was part of the Philadelphia combined statistical area from 1990 until 2000.

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".