Here's a look at what you need to know about anthrax, an acute infectious disease caused by bacterium Bacillus anthracis.
Facts: There are three types of anthrax infection: cutaneous (through the skin), inhalation (through the lungs; the most deadly) and gastrointestinal (through digestion).
It can be contracted by handling products from infected animals or by breathing in anthrax spores and by eating undercooked meat from infected animals.
Anthrax has been blamed for several plagues over the ages that killed both humans and livestock; emerged in World War I as a biological weapon.
The CDC categorizes anthrax as a Category A agent: one that poses the greatest possible threat for a negative impact on public health; one that may spread across a large area or need public awareness and requires planning to protect the public's health.
Amerithrax: Five people died and 17 people sickened during anthrax attacks in the fall of 2001; outbreak referred to as Amerithrax.
Anthrax was sent via anonymous letters to news agencies in Florida and New York and a congressional office building in Washington.
Of the five victims who died of inhalation anthrax, two were postal workers. The other three victims were an elderly woman from rural Connecticut, a Manhattan hospital worker from the Bronx and an employee at a Florida tabloid magazine who may have contacted anthrax through cross-contamination.
The letters were sent to NBC anchor Tom Brokaw, Sen. Majority Leader Tom Daschle, Sen. Patrick Leahy, and the New York Post offices. The letters were postmarked Trenton, New Jersey.
No arrests have been made in the attacks.
The FBI has interviewed 9,100 people and issued 6,100 subpoenas in the case.
4.8 million masks and 88 million gloves were purchased by the Postal Service for its employees, and 300 postal facilities were tested for anthrax.
Over 32,000 people took antibiotics after possible exposures to anthrax.
Victims: Lundgren, Ottilie - Connecticut woman, dies of inhalation anthrax, November 22, 2001
Nguyen, Kathy - employee at Manhattan hospital, dies of inhalation anthrax, October 31, 2001
Curseen, Joseph Jr. - DC area postal worker, dies of inhalation anthrax, October 22, 2001
Morris, Thomas Jr. - DC postal worker, dies of inhalation anthrax, October 21, 2001
Stevens, Bob - photo editor at American Media Inc, dies of inhalation anthrax, October 5, 2001
Timeline: October 5, 2001 - Sun photo editor Bob Stevens dies of inhalation anthrax.
October 12, 2001 - NBC News announces that an employee has contracted anthrax.
October 15, 2001 - A letter postmarked Trenton, New Jersey, opened by an employee of Senate Majority Leader Tom Daschle contains white powdery substance later found to be "weapons grade" strain of anthrax spores. More than two dozen people in Daschle's office tested positive for anthrax after the envelope was discovered.
October 19, 2001 - An unopened letter tainted with anthrax is found in the offices of the New York Post. One Post employee is confirmed to have a cutaneous infection and a second shows symptoms of the same infection.
October 21, 2001 - DC postal worker Thomas Morris Jr. dies of inhalation anthrax.
October 22, 2001 - DC postal worker Joseph Curseen dies of inhalation anthrax.
October 31, 2001 - Kathy Nguyen, a stockroom worker for the Manhattan Eye, Ear and Throat Hospital, dies of inhalation anthrax.