Raymond Chen from Microsoft has been saving all his spam and virus mails since 1997 and created a graphical representation of them: a dot per mail in time/size coordinates.
Some of his observations:
Spam went ballistic starting in 2002. You could see it growing in 2001, but 2002 was when it really took off. The big red splotch in August 2003 around the 100K mark is the Sobig virus. The horizontal line in 2004 that wanders around the 2K mark is the Netsky virus.
Source: Visual history of spam
But no worries, Bill Gates promised to terminate spam by 2006.