Malware accounted for one in every 290 emails in February, according to the latest report from MessageLabs, and the study's authors say it's getting worse, not better.

The most popular vehicle for delivering malicious software in February was to conceal the threat within a PDF file.

MessageLabs estimated that 65 percent of targeted attacks in 2010 used a PDF exploit, up 12.4 percent from the 2009 market of 52.6 percent.

"Despite a downturn this month, if the trend were to continue as it has over the past year, 76 percent of targeted malware could be used for PDF-based attacks by mid-2011," according to the Symantec (NASDAQ: SYMC) MessageLabs Intelligence report, released Tuesday.

MessageLabs' estimate that one in 290.1 emails is malicious constitutes an increase of 0.07 percentage points since January 2010. While it may seem small, it's an area where a small mistake can multiply quickly.

"In February, 0.345 percent [of all email] was malicious, making February among the most prolific time periods in terms of simultaneous attacks and malware family integration across Zeus (aka Zbot), Bredolab and SpyEye," the report said.

Meanwhile, spam, which had reached its lowest numbers in nearly two years in the January report, reversed direction in February.

Whereas spam dropped in January to 78.6 percent of emails, a decline of some 3.1 percentage points from December, in February spam reached 81.3 percent, which constituted an increase of 2.7 percentage points since January 2011, the February report said.

Additionally, the February report suggests that similarities among the Zeus, Bredolab and SpyEye malware "families" and the apparent timing of their virus attacks may suggest that malware developers are working together.

"It is particularly interesting that these malware families are used in these synchronized, integrated attacks, because at least until recently they have been bitter rivals," the report said.

The report is available as PDF format here.

Stuart J. Johnston is a contributing writer at, the news service of, the network for technology professionals. Follow him on Twitter @stuartj1000.