by Paul Jay, CBCNews.ca

For most computer users, spam is recognized and deleted from the inbox almost as quickly as it arrives, but for reasons that continue to escape the general population, spam seems to always be on the rise.

That's why the latest reports from computer security firms include an odd, and possibly encouraging fact: After a brief spike in .pdf spam, the tactic appears to have gone away.

According to Symantec's Sept. report, spam using Portable Document Format (or PDF) files spiked in August, making up 30 per cent of all spam messages sent on Aug. 7. But by the end of the month, the files were found on less than one per cent of spam, according to security firms Sophos and Symantec.

Sophos security analyst Ron O'Brien told Networkworld of one possible explanation: that opening a .pdf is an extra step, increasing the likelihood that most users just wouldn't bother.

"The most likely reason for the drop-off is that the campaigns were largely unsuccessful," he told the trade newsweekly.

Take note, gullible people of the world: Abstinence is the only prevention.