Every good statistician knows that correlation does not imply causation.  Just because two things appear linked does not mean they are. However, with historical data we often don't have the luxury of carrying out controlled, scientific experiments to see if A really does cause B.

One particularly interesting example is the possible link between influenza and coronary heart-disease (CHD) mortality. The following chart is derived from data from the Massachusetts Department of Public Health Registry of Vital Records and Statistics, and is inspired by a similar chart by Azambuja and Levins (2007):

While we know that correlation does not imply causation, there does seem to be a rising incidence of CHD along…