Virus volume in spam e-mail increased 10% from the same quarter in 2009. Virus levels increased 111% between August 2009 and August 2010, with 188 million viruses blocked in a single day a record. E-mail viruses are taking advantage of previously sent e-mails harvested from the hard drives of infected computers to fool spam filters. Spammers may be trying to build up botnets in advance of the holiday season, when many users go online to purchase gifts, and spammers are more likely to find success pushing their own wares.
Source: https://threatpost.com/spammers-gearing-botnets-holiday-rush-101910/74585/