Google reports that spam is down on last year, but contains more viruses than it did a year ago.
Third-quarter spam levels are down 24% from last year, while virus levels have increased 10%.
Spam is down 16% down from the second quarter of 2010, while virus levels jumped 42% in third quarter.
Google e-mail security and archiving services, powered by Postini, reported a 241% increase in virus volume in August from the month before and a record 188 million viruses blocked in a single day.
Adrian Soghoian and Adam Hollman of the Google Postini Services Team believe this is a sign that spammers are building up their botnets for the end-of-year holiday season.
"The spike in malware attacks during August suggests that we might see higher levels of spam moving forward into Q4 as botnet "seeds" planted during this time begin to take action," they wrote in a blog post.
The growth of botnets is supported by research from Microsoft, which shows the firm's anti-malware tools cleaned 6.5 million botnet infections between April and June this year - double the number in the same period last year.
"The user is often unaware of the attacks, making these messages particularly dangerous and difficult to detect," they wrote.
According to Soghoian and Hollman, spammers have begun copying the text of sent e-mails stored on infected computers with the hope that spam composed of actual sent messages can bypass web filters and trick recipients.
Spammers are also using an increasing number of URL shortening services such as bit.ly to conceal URLs that recipients might otherwise avoid.
"A shortened URL sent from a 'friend' might seem innocuous enough, but, as always, links and e-mails sent from unknown senders should be scrutinised before further action is taken," they wrote.