(CNN)Here's what you need to know about anthrax, an acute infectious disease caused by bacterium Bacillus anthracis.
There are three types of anthrax infection: cutaneous (through the skin), inhalation (through the lungs; the most deadly) and gastrointestinal (through digestion). There has been a fourth type of anthrax identified as injection anthrax. This is common in heroin-injecting users in northern Europe. This has never been reported in the United States.
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. It 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.
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 contracted anthrax through cross-contamination.
No arrests have been made in the attacks.
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 exposure to anthrax.
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
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 test positive for anthrax after the envelope is 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.
November 9, 2001 - The FBI releases a behavioral profile of the suspect, who is probably a male loner and might work in a laboratory.
November 16, 2001 - A letter sent to Sen. Patrick Leahy is found to contain anthrax. The letter is among those at the Capitol that has been quarantined. The letter contains at least 23,000 anthrax spores and is postmarked October 9, in Trenton, New Jersey.
November 22, 2001 - Ottilie Lundgren, a 94-year-old Connecticut woman, dies of inhalation anthrax.
January 2002 - FBI agents interview former U.S. Army bioweapons scientist Steven Hatfill as part of the anthrax investigation.
June 2002 - Bioweapons researcher Steven Hatfill is named a "person of interest" by the FBI.
June 25, 2002 - The FBI searches Steven Hatfill's Maryland apartment and Florida storage locker with his consent.
June 27, 2002 - The FBI says it is focusing on 30 biological weapons experts in its probe.
August 1, 2002 - The FBI uses a criminal search warrant to search Steven Hatfill's Maryland apartment and Florida storage locker a second time; anthrax swab tests come back negative.
August 6, 2002 - Attorney General John Ashcroft refers to Hatfill as a "person of interest."
August 11, 2002 - Steven Hatfill holds a press conference declaring his innocence, and a second one on August 25, 2002.
September 11, 2002 - The FBI searches Hatfill's former apartment in Maryland for the third time.
August 26, 2003 - Hatfill files a civil lawsuit against Attorney General John Ashcroft, the Justice Department and the FBI saying his constitutional rights have been violated. The suit alleges violations of Hatfill's Fifth Amendment rights by preventing him from earning a living, violations of his Fifth Amendment rights by retaliating against him after he sought to have his name cleared in the anthrax probe and the disclosure of information from his FBI file. The suit, which names the Justice Department, FBI, Attorney General John Ashcroft and various lower level Justice and FBI officials, asks for a declaration that government officials violated Hatfill's constitutional rights and seeks an injunction against future violations. The suit also seeks an undetermined amount of monetary damages.
July 11, 2004 - The former headquarters of American Media, Inc. in Boca Raton, Florida where Bob Stevens contracted the anthrax is pumped full of chlorine dioxide gas for decontamination, the last building in the country exposed to anthrax in the fall of 2001.
March 14, 2005 - Pentagon officials announce that sensors have detected anthrax in two Pentagon mail facilities. The facilities have been closed and nearly 300 workers have been tested for exposure to the bacteria.
June 27, 2008 - The Justice Department reaches a settlement with former Army scientist Steven Hatfill. The settlement requires the Justice Department to pay Hatfill a one-time payment of $2.825 million and to buy a $3 million annuity that will pay Hatfill $150,000 a year for 20 years. In return, Hatfill drops his lawsuit, and the government admits no wrongdoing.
July 29, 2008 - Bruce Ivins, a former researcher at the Army's bioweapons laboratory at Fort Detrick, Maryland, dies after overdosing during a suicide attempt on July 27.
August 6, 2008 - Judge unseals and releases hundreds of documents in the 2001 FBI Anthrax investigation.
August 8, 2008 - The Justice Department formally exonerates Hatfill.
September 25, 2008 - Court releases more documents including e-mails that Bruce Ivins sent to himself.
February 19, 2010 - The Justice Department , FBI and U.S. Postal Inspection Service announce their investigation into the 2001 anthrax mailings is at an end.
March 23, 2011 - A report, entitled The Amerithrax Case, is released through the Research Strategies Network, a non-profit think tank based in Virginia. According to the report, old mental health records suggest Bruce Ivins should have been prevented from holding a job at a U.S. Army research facility in Maryland. The report was requested by the U.S. Department of Justice.
October 9, 2011 - The New York Times publishes an article saying there are scientists questioning the FBI assertions regarding Bruce Ivins. Possibly Ivins, if he was involved, worked with a partner. Also, the scientists say the presence of tin in the dried anthrax warrants that the investigation be reopened.
November 23, 2011 - The Justice Department settles for $2.5 million with the family of Bob Stevens, the first victim to die in the 2001 anthrax attack. The family originally sued for $50 million in 2003, arguing that the military laboratory should have had tighter security.
June 19, 2014 - The Centers for Disease Control and Prevention announces that approximately 86 employees may have been unintentionally exposed to anthrax.
July 11, 2014 - The Centers for Disease Control and Prevention holds a news conference to discuss the conclusion of its investigation. The CDC says that while it is "not impossible" that the staff was exposed to viable B. anthracis (anthrax), it is "extremely unlikely" that this happened.