An open question on email

I have been comparing email statistics between two years ago and this week. The percentages of emails rejected as viruses seems to have changed dramatically. Two years ago, the number of virus emails flagged by ClamAV made up at least 10% (and as high as 30%) of the email volume. This week, I'm seeing less than 1%.

I have no reason to doubt that ClamAV is performing normally. However, such a change is still unexpected. Has anyone else noticed a decrease (at least percentage-wise) of virus emails?