Google reports that it's seeing less spam than it did a year ago, but that's where the good news ends. The spam that is being sent contains more viruses.

Spam levels are down 24% from the third quarter of 2009 and 16% from the second quarter of 2010 while virus levels jumped 10% and 42% during the same periods respectively.

August was a particularly bad month for viruses, setting a record at Postini for the highest number of viruses blocked in a single day: 188 million.

Adrian Soghoian and Adam Hollman, members of Google's Postini services team, see this as a sign of an upcoming spam surge.

"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," wrote Soghoian and Hollman in a blog post on Monday.

Botnets continue to proliferate, despite recent botnet takedowns. Last week, Microsoft noted that during the second quarter of 2010 it identified 2.2 million bot-infected computers in the U.S., four times as many as it found in Brazil, the country with the second highest number of bot-infected computers.

Soghoian and Hollman note that spammers have begun copying the text of sent e-mails stored on infected computers in the hope that spam composed of actual sent messages finds its way through filters more easily and dupes more recipients than typical spam texts promising access to lost fortunes or the like.

They also observed an increase in the volume of e-mail messages containing shortened URLs. URL shortening services like bit.ly or goo.gl, recently launched by Google, are popular with spammers because they conceal URLs that message recipients might otherwise find suspicious.