In its July 2011 Symantec Intelligence Report, Symantec said that "polymorphic malware", or malware that is aggressive and rapidly changes, was seen in 23. 7 percent of e-mails identified as malicious. This figure is more than double seen six months ago, Symantec added, "indicating a much more aggressive strategy on the part of the cyber criminals responsible."

In July 2011, the global ratio of spam in e-mail traffic rose to 77.8 percent, indicating that one in 1.29 e-mails is a spam. This represents an increase of 4.9 percentage points compared with June 2011.

"The number of variants, or different strains of malware involved in each attack has grown dramatically, by a factor of 25 times, when compared to the previous six months. This is a disturbing proliferation in such a short time, increasing the risk profiles of many organisations as these new strains are much harder to detect using traditional security defences," said Paul Wood, senior intelligence analyst, Symantec.cloud.

The aggressive malware was frequently in attached ZIP files, and often disguised as a PDF file or an office productivity document.