Acommon theory about influenza is that it is a disease occurring primarily in cold weather. doctors report incidences of
illness such as influenza to the centers for disease control (cdc). the graph below shows data for three years. note that
this graph shows from october of one year to october of the following year. the leftmost data on the graph begins with
week 40 (october) and shows data through the end of the year. the check mark for week 1 (beginning of january) is
roughly in the middle of the x-axis.
percentage of visits for influenza-like illness from 2006 to 2009
percentage of visits for ili
key:
. - 2006 - 2007
= 2007 - 2008
= 2008 - 2009
national average
"there was no week 53 during
the 2006-2007 and 2007-2008
seasons, therefore the week 53
data point for those