According to an analysis by security vendor Postini, of 150 billion e-mail messages sent in 2005, e-mail-borne viruses represented 2.5 percent of all inbound e-mail messages.
The reason for the increase is partially attributable to the Sober virus, which Postini reports was the largest virus outbreak it has ever recorded.
In the four-week period between Thanksgiving and Christmas, Postini reported that it blocked 1.2 billion e-mails that contained the sober virus. Overall, Sober represented 46 percent of all intercepted viruses by Postini in 2005.
The extra half to 1 percent jump to 2.5 percent for the year came just from the year-end Sober outbreak. A recent study from IBM reported that 2.8 percent of all e-mails in 2005 contained a virus.
I think we were very surprised to see that, in this day and age, there could be a worm that virulent that could propagate itself to so many computers," Lochart told internetnews.com.
"It's easy for us to assume working in the corporate industry that all PCs have some form of anti-virus protection, and I think we forget about the fact that, in emerging markets and here in the U.S., we still have so many new computer users -- new always-on broadband computers.
"A hacker that writes a nice piece of code can get a virus to propagate itself in an unbelievable fashion."
The Sober outbreak also affected the overall percentage of spam in e-mail traffic, which hovered between 75 and 80 percent for the year.