A common 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.

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