Spam also dropped in December which might be relevant to the decrease of email viruses. The drop might be short lived though: http://www.bbc.co.uk/news/technology-12126880

"The amount of junk e-mail being sent across the globe has seen a dramatic fall in recent months.

The volume of spam has dropped steadily since August, but the Christmas period saw a precipitous decline.

One security firm detected around 200 billion spam messages being sent each day in August, but just 50 billion in December.

While the reasons for the decline are not fully understood, spam watchers warn the lull may not last."

And: http://www.bbc.co.uk/news/technology-12154118