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

The 2002 anthrax attacks, also known as Amerithrax (a portmanteau of "America" and "anthrax", from its FBI case name), occurred in the United States over the course of several weeks beginning on September 18, 2001, one week after the September 11 terrorist attacks. Letters containing anthrax spores were mailed to several news media offices and to Democratic Senators Tom Daschle and Patrick Leahy, killing five people and infecting 17 others. According to the FBI, the ensuing investigation became "one of the largest and most complex in the history of law enforcement".A major focus in the early years of the investigation was bioweapons expert Steven Hatfill, who was eventually exonerated. Bruce Edwards Ivins, a scientist at the government's biodefense labs at Fort Detrick in Frederick, Maryland, became a focus around April 4, 2005. On April 11, 2007, Ivins was put under periodic surveillance and an FBI document stated that he was "an extremely sensitive suspect in the 2001 anthrax attacks". On July 29, 2008, Ivins committed suicide with an overdose of acetaminophen (Tylenol).Federal prosecutors declared Ivins the sole perpetrator on August 6, 2008, based on DNA evidence leading to an anthrax vial in his lab. Two days later, Senator Chuck Grassley and Representative Rush D. Holt, Jr. called for hearings into the Department of Justice and FBI's handling of the investigation. The FBI formally closed its investigation on February 19, 2010.In 2008, the FBI requested a review of the scientific methods used in their investigation from the National Academy of Sciences, which released their findings in the 2011 report Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters. The report cast doubt on the government's conclusion that Ivins was the perpetrator, finding that the type of anthrax used in the letters was correctly identified as the Ames strain of the bacterium, but that there was insufficient scientific evidence for the FBI's assertion that it originated from Ivins's laboratory. The FBI responded by pointing out that the review panel asserted that it would not be possible to reach a definite conclusion based on science alone, and said that a combination of factors led the FBI to conclude that Ivins had been the perpetrator. Some information is still sealed concerning the case and Ivins's mental health.:8 footnote The government settled lawsuits that were filed by the widow of the first anthrax victim Bob Stevens for $2.5 million with no admission of liability. The settlement was reached solely for the purpose of "avoiding the expenses and risks of further litigations", according to a statement in the agreement.

Anthrax is an infection caused by the bacterium Bacillus anthracis. It can occur in four forms: skin, lungs, intestinal and injection. 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 (which may contain blood), abdominal pains, nausea and vomiting. The injection form presents with fever and an abscess at the site of drug injection.According to the Centers for Disease Control the first clinical descriptions of cutaneous anthrax were given by Maret in 1752 and Fournier in 1769. Before that anthrax had been described only through historical accounts. The Prussian scientist Robert Koch (1843–1910) was the first to identify Bacillus anthracis as the bacterium that causes anthrax.

Anthrax is spread by contact with the bacterium's spores, which often appear in infectious animal products. Contact is by breathing or eating or through an area of broken skin. It does not typically spread directly between people. Risk factors include people who work with animals or animal products, travelers and military personnel. Diagnosis can be confirmed by finding antibodies or the toxin in the blood or by culture of a sample from the infected site.Anthrax vaccination is recommended for people at high risk of infection. Immunizing animals against anthrax is recommended in areas where previous infections have occurred. A two-month course of antibiotics such as ciprofloxacin, levofloxacin and doxycycline after exposure can also prevent infection. If infection occurs, treatment is with antibiotics and possibly antitoxin. The type and number of antibiotics used depend on the type of infection. Antitoxin is recommended for those with widespread infection.A rare disease, human anthrax is most common in Africa and central and southern Asia. It also occurs more regularly in Southern Europe than elsewhere on the continent and is uncommon in Northern Europe and North America. Globally, at least 2,000 cases occur a year, with about two cases a year in the United States. Skin infections represent more than 95% of cases. Without treatment the risk of death from skin anthrax is 23.7%. For intestinal infection the risk of death is 25 to 75%, while respiratory anthrax has a mortality of 50 to 80%, even with treatment. Until the 20th century anthrax infections killed hundreds of thousands of people and animals each year. Anthrax has been developed as a weapon by a number of countries. In herbivorous animals infection occurs when they eat or breathe in the spores while grazing. Animals may become infected by killing and/or eating infected animals.