Spam levels continued to rise in April, reaching the highest level in 19 months as image spam made a comeback and keywords related to the outbreak of swine flu became more common, according to security firms.

Unsolicited e-mail accounted for more than 85 percent of all e-mail messages sent in April, a leap of nearly 10 percent since March, according to the latest spam report (pdf) from e-mail security firm MessageLabs, which — like SecurityFocus — is part of Symantec. The jump continued a resurgence in junk e-mail following a severe drop in spam that accompanied the takedown of hosting provider McColo in November. Many researchers had maintained that McColo hosted spam networks and botnet command-and-control servers, and the drop in spam supported those assertions.

However, the volume of junk e-mail quickly resumed its climb. In addition, spammers appear to be able to maintain sustained operations, a change from previous years, said Paul Wood, a senior analyst with MessageLabs.

"In the past, if you had a big burst of spam, it would take a few hours and then die back down," he said. "But now we are seeing more frequent runs lasting a lot longer."

Image spam, which used to be used by fraudsters to dodge anti-spam defenses, has been resurrected, Wood said. Yet, rather than being included as an e-mail attachment, the images are typically hosted on a compromised servers, he said.

In an almost-inevitable trend, spammers have joined the rest of the world in their interest in the latest influenza epidemic. The occurrence of the words "swine flu" in the subject lines of junk e-mail messages has skyrocketed, McAfee's security and communications manager David Marcus said in a blog post.

"Many people may not realize that the words “swine” and “flu” had really not been seen in spam before this past weekend and almost certainly not together in the same subject line," Marcus said.

MessageLabs confirmed that flu-related spam had risen recently, but oddly, the spam has dropped off in the last day, researcher Wood said.

"They have almost gone away today," he said. "Whether the spammers have gone on a hiatus or not, the (flu) spam has almost gone away."

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