Worldwide spam levels have mysteriously dropped off over the past week, according to managed email provider SoftScan, possibly as a result of a major botnet going out of service.

Spam levels continued to rise in December, but crashed by 30 percent in the first week of January, SoftScan said on Tuesday. The company has seen nothing similar in the past, but believes the most likely explanation is that a botnet - a network of compromised machines - has temporarily lost control of its client systems.

The results are based on SoftScan's analysis of the email systems it scans for about half a million users, mostly international businesses, according to chief technology officer Diego d'Ambra.

"This is very unusual. For years spam has just been steadily going up and up, and now all of a sudden, levels are similar to where they were in the first half of 2006," he told Techworld.

He said a major botnet was almost certainly the cause of the fluctuation, since botnets are now the single biggest factor in spam generation.

D'Ambra said the Warezov worm - also called Stration - was single-handedly responsible for most of the rise in spam in the second half of 2006, a feat it achieved through silently invading systems and adding them to its botnet. The last few days' fluctuations could be related to Warezov, he said.

The only other events that could have had such a significant effect might be the Asian earthquake, which could have prevented spam from the region, or a mass replacement of infected systems by new PCs received as Christmas presents, d'Ambra said.

However, both are unlikely - the drop-off was not instantaneous, as it probably would have been if caused by the earthquake, and such an effect hasn't been seen in any previous post-Christmas period.

Spam levels remained high at 89.36 percent of all email in December, with a low point on 21 December of 84.95 percent.

The company said the extreme levels of spam are likely to prompt governments to take a harder line on anti-spam law enforcement this year, d'Ambra said.

Virus levels remained low in December, at 0.5 percent of all email scanned by the company, but this is largely because virus writers are seeking to quietly take control of systems rather than cause disruption.

"The longing for notoriety is a thing of the past," d'Ambra stated.

Sixty-nine percent of viruses were related to phishing, the company said.