<prompt> Answer this question truthfullyWhat is the historical background of influenza?  <response>: <reason> During October 1, 2006--May 19, 2007, the WHO and the National Respiratory and Enteric Virus Surveillance System NREVSS collaborating laboratories in the United States tested 179,268 respiratory specimens for influenza viruses; 23,753 13.2 were positive. Of these, 18,817 79.2 were influenza A viruses and 4,936 20.8 were influenza B viruses. Among the influenza A viruses, 6,280 33.4 were subtyped; 3,912 62.3 were influenza A H1 viruses and 2,368 37.7 were influenza A H3 viruses. The proportion of specimens testing positive for influenza first exceeded 10 during the week ending December 23, 2006 week 51, peaked at 28.0 during the week ending February 10, 2007 week 6, and declined to less than 10 during the week ending April 28, 2007 week 17. The proportion was above 10 positive for 14 consecutive weeks. The peak percentage of specimens testing positive for influenza during the previous three seasons ranged from 22.6 to 34.7, and the peak occurred during early December to early March. During the previous three influenza seasons, the number of consecutive weeks during which more than 10 of specimens tested positive for influenza ranged from 13 to 17 weeks.During September 30, 2007--May 17, 2008, the WHO and the National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 225,329 specimens for influenza viruses; 39,827 18 were positive. Of the positive specimens, 28,263 71 were influenza A viruses, and 11,564 29 were influenza B viruses. Among the influenza A viruses, 8,290 29 were subtyped; 2,175 26 were influenza A H1N1, and 6,115 74 were influenza A H3N2 viruses. The proportion of specimens testing positive for influenza first exceeded 10 during the week ending January 12, 2008 week 2, peaked at 32 during the week ending February 9, 2008 week 6, and declined to 10 during the week ending April 19, 2008 week 16. The proportion positive was above 10 for 14 consecutive weeks. The peak percentage of specimens testing positive for influenza during the previous three seasons ranged from 22 to 34 and the peak occurred during mid-February to early March. During the previous three influenza seasons, the number of consecutive weeks during which more than 10 of specimens tested positive for influenza ranged from 13 to 17 weeksFrom September 28, 2008, to April 4, 2009, the WHO and the National Respiratory and Enteric Virus Surveillance System NREVSS collaborating laboratories in the United States tested 173,397 respiratory specimens for influenza viruses, 24,793 14.3 of which were positive. Of these, 16,686 67.3 were positive for influenza A viruses, and 8,107 32.7 were positive for influenza B viruses. Of the 16,686 specimens positive for influenza A viruses, 6,735 40.4 were subtyped by real-time reverse transcription-polymerase chain reaction or by virus culture; 6,049 89.8 of these were influenza A H1N1 viruses, and 686 10.2 were influenza A H3N2 viruses. The percentage of specimens testing positive for influenza first exceeded the seasonal threshold of 10 during the week ending January 17, 2009, and peaked at 25.0 during the week ending February 14, 2009. For the week ending April 4, 2009, 12.3 of specimens tested for influenza were positive. The relative proportion of influenza B viruses increased during February and March, and since the week ending March 14, 2009, 50 of the positive influenza specimens have been influenza B.Since April 2009, the beginning of the 2009 H1N1 pandemic, through June 12, 2010, approximately 740,000 influenza specimens were tested for influenza, and the number of laboratory-confirmed positives was approximately four times the average of the previous four seasons. Two peaks in percentage of specimens testing positive for influenza occurred: 43.1 in June during the initial pandemic wave, and 38.2 in October during the second wave. During August 30, 2009--June 12, 2010, the 2009--10 influenza season, the WHO and National Respiratory and Enteric Virus Surveillance System NREVSS collaborating laboratories in the United States tested 468,218 specimens for influenza viruses; 91,152 19.5 were positive. The proportion of specimens testing positive for influenza during the 2009--10 season exceeded 20 during the week ending August 30, 2009, peaked at 38.2 during the week ending October 24, and declined to less than 10 during the week ending December 12. Of the 91,152 positive specimens from 2009-10 season, 90,758 99.6 were influenza A viruses and 394 0.4 were influenza B viruses. Among the influenza A viruses, 67,022 73.8 were subtyped; 66,916 99.8 were 2009 pandemic H1N1, 72 0.1 were influenza A H3N2, and 34 0.1 were seasonal influenza A H1N1 viruses.During October 3, 2010--May 21, 2011, the WHO and National Respiratory and Enteric Virus Surveillance System NREVSS collaborating laboratories in the United States tested 246,128 specimens for influenza viruses; 54,226 22 were positive. Of the positive specimens, 40,282 74 were influenza A viruses, and 13,944 26 were influenza B viruses. Among the influenza A viruses, 28,545 71 were subtyped; 17,59962 were influenza A H3N2 viruses, and 10,946 38 were 2009 influenza A H1N1 viruses. The proportion of specimens testing positive for influenza during the 2010-11 season first exceeded 10, indicating higher levels of virus circulation, during the week ending November 27, 2010. The proportion peaked at 36 during the week ending February 5, 2011, and declined to 10 during the week ending April 16, 2011.During October 2, 2011May 19, 2012, the WHO and National Respiratory and Enteric Virus Surveillance System NREVSS collaborating laboratories in the United States tested 169,453 specimens for influenza viruses; 22,417 13 were positive. Of the positive specimens, 19,285 86 were influenza A viruses, and 3,132 14 were influenza B viruses. Among the influenza A viruses, 14,968 78 were subtyped; 11,002 74 were influenza A H3N2 viruses, and 3,966 26 were pH1N1 viruses. The proportion of specimens testing positive for influenza during the 201112 season first exceeded 10 indicating higher levels of viral circulation during the week ending February 4, 2012, and peaked at 32 during the week ending March 17, 2012.During September 30, 2012May 18, 2013, the WHO and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 311,333 specimens for influenza viruses; 73,130 23 were positive. Of the positive specimens, 51,675 71 were influenza A viruses, and 21,455 29 were influenza B viruses. Among the seasonal influenza A viruses, 34,922 68 were subtyped; 33,423 96 were influenza A H3N2 viruses, and 1,497 4 were pH1N1 viruses. In addition, two variant influenza A H3N2v viruses were identified. Typically the influenza season is said to begin when certain key indicators remain elevated for a number of consecutive weeks. One of these indicators is the percent of respiratory specimens testing positive for influenza. The proportion of specimens testing positive for influenza during the 201213 season first exceeded 10 during the week ending November 10, 2012 week 45, and peaked at 38 during the week ending December 29, 2012 week 52. 