(CNN)Here's some background information about concussions in the NFL. Reports show an increasing number of retired NFL players who have suffered concussions developed memory and cognitive issues such as dementia, Alzheimer's, depression and chronic traumatic encephalopathy (CTE).
A concussion is a type of traumatic brain injury caused by a blow to the head.
Most concussions occur without losing consciousness.
Chronic traumatic encephalopathy (CTE) is a degenerative disease of the brain and is associated with repeated head traumas like concussions.
Among the plaintiffs in concussion-related lawsuits: Art Monk, Tony Dorsett, Jim McMahon, and Jamal Anderson.
Common Symptoms of Concussions: (The NFL Player Concussion Pamphlet)
Loss of consciousness
2012 - 261 diagnosed concussions during preseason and regular-season practices and games combined.
2013 - 229 diagnosed concussions during preseason and regular-season practices and games combined.
2014 - 202 diagnosed concussions during preseason and regular-season practices and games combined.
1994 - NFL Commissioner Paul Tagliabue creates the Mild Traumatic Brain Injury Committee. Dr. Elliot Pellman is named chairman despite not having experience with brain injuries.
2002 - Dr. Bennet Omalu, a forensic pathologist and co-founder of the Brain Injury Research Institute, identifies chronic traumatic encephalopathy (CTE) in the brain of former Pittsburgh Steelers' center Mike Webster, 50, who committed suicide. Omalu is the first to identify CTE in American football players.
January 2005 - The NFL's Mild Traumatic Brain Injury Committee finds that returning to play after sustaining a concussion "does not involve significant risk of a second injury either in the same game or during the season."
2005 and 2006 - Dr. Omalu identifies chronic traumatic encephalopathy (CTE) in the brains of former Pittsburgh Steelers players Terry Long, 45, and Andre Waters, 44. Both had committed suicide.
February 2007 - Dr. Elliot Pellman steps down as chairman of the Mild Traumatic Brain Injury Committee but remains on the committee.
June 2007 - The NFL holds a medical conference on concussions.
August 14, 2007 - The NFL formalizes new concussion guidelines which include a telephone hotline to report when a player is being forced to play contrary to medical advice.
October 28, 2009 - Part I of the House Judiciary Committee hearing on Legal Issues Relating to Football Head Injuries. NFL Commissioner Roger Goodell defends the League's policy regarding concussions.
January 4, 2010 - Part II of the House Judiciary Committee hearing on Legal Issues Relating to Football Head Injuries. Dr. Ira Casson, one of the co-chairs of the Mild Traumatic Brain Injury Committee, denies a link between repeat head impacts and long-term brain damage.
March 2010 - The NFL's Mild Traumatic Brain Injury Committee is renamed the Head, Neck and Spine Committee. Two new co-chairs are selected, and Dr. Elliot Pellman is no longer a member of the panel.
October 20, 2010 - NFL Commissioner Goodell issues a memo to all 32 teams that warns of possible suspensions for offenders that violate the "playing rules that unreasonably put the safety of another player in jeopardy have no place in the game, and that is especially true in the case of hits to the head and neck."
February 17, 2011 - Former Chicago Bears defensive back Dave Duerson, 50, commits suicide with a gunshot wound to the chest rather than his head so his brain can be researched for chronic traumatic encephalopathy (CTE). Boston University researchers find CTE in Duerson's brain, the same disease found in other deceased NFL players.
April 19, 2012 - Former Atlanta Falcons safety Ray Easterling, 62, commits suicide. An autopsy finds signs of chronic traumatic encephalopathy (CTE). Easterling had been a plaintiff in a class action lawsuit against the NFL over concussion-related injuries filed in August 2011.
May 2, 2012 - Former NFL linebacker Junior Seau, 43, is found dead with a gunshot wound to the chest, classified as a suicide. Friends and family members say the suicide was brought on by multiple concussions, but an initial autopsy report finds no apparent brain damage. Portions of Seau's brain have been sent to the National Institutes of Health for further study.
June 7, 2012 - A unified lawsuit combining more than 80 concussion-related lawsuits on behalf of more than 2,000 National Football League players is filed in federal court in Philadelphia. The players accuse the NFL of negligence and failing to notify players of the link between concussions and brain injuries, in Multi-district Litigation Case No. 2323.
August 30, 2012 - The NFL files a motion to dismiss the concussion related lawsuits filed by former players.
September 5, 2012 - The Foundation for the National Institutes of Health announces that the NFL has committed to donating $30 million to support research on medical conditions prominent in athletes.
January 10, 2013 - The National Institutes of Health releases the results of their analysis of Junior Seau's brain tissue confirming that Seau did suffer from chronic traumatic encephalopathy (CTE).
January 23, 2013 - Junior Seau's family files a wrongful death lawsuit against the NFL, claiming that Seau's suicide was the result of a brain disease caused by violent hits he endured while playing the game.
August 29, 2013 - The NFL and ex-players reach a deal in the class action lawsuit that calls for the NFL to pay $765 million to fund medical exams, concussion-related compensation, medical research for retired NFL players and their families, and litigation expenses, according to a court document filed in U.S. District Court in Philadelphia. The agreement still needs to be approved by the judge assigned to the case, which has grown to include more than 4,500 plaintiffs.
December 13, 2013 - The body of former NFL linebacker Jovan Belcher is exhumed in order to perform tests on his brain, a lawyer for the player's family tells the Kansas City Star. On December 1, 2012, Belcher, 25, shot his longtime girlfriend to death and then killed himself.
December 2013 - Ryan Freel is the first Major League Baseball player to be diagnosed with chronic traumatic encephalopathy (CTE), according to researchers at the Boston University School of Medicine. Freel committed suicide in December 2012 at the age of 36.
January 14, 2014 - A federal judge declines to approve a proposed $760 million settlement of claims arising from concussions suffered by NFL players, saying she didn't think it was enough money.
May 28, 2014 - Former Miami Dolphins quarterback Dan Marino, and 14 other former NFL players, sues the NFL over concussions. The lawsuit claims the NFL knew for years of the link between concussions and long-term health problems.
June 3, 2014 - It is reported that Dan Marino has withdrawn his name from the concussion lawsuit.
July 7, 2014 - The U.S. District Court in Pennsylvania grants preliminary approval to a settlement between retired NFL players and the National Football League.
July 17, 2014 - Former NFL players Christian Ballard and Gregory Westbrooks file suit against the NFL Players Association, alleging the union withheld information about head injuries.
September 30, 2014 - Dr. Piotr Kozlowski releases a report on former NFL linebacker Jovan Belcher, stating that he likely had CTE when he killed his girlfriend and himself in 2012.
April 22, 2015 - A federal judge gives final approval to a class-action lawsuit settlement between the National Football League and thousands of former players. The agreement provides up to $5 million per retired player for serious medical conditions associated with repeated head trauma.
November 25, 2015 - Frank Gifford's family says he suffered from CTE. Gifford's diagnosis comes amid a growing focus on the risks athletes face from suffering repeated concussions, and just hours after the NFL admitted its concussion protocols had failed when St. Louis Rams quarterback Case Keenum kept playing Sunday even after his head slammed into the field.
February 3, 2016 - Former Oakland Raiders quarterback Ken Stabler, who died in July 2015 of colon cancer, is diagnosed posthumously with CTE by researchers at Boston University.