MessageLabs, a managed email security provider, has published their Spam and Virus stats for 2004. Their anti-spam system scanned over 12.6 billion emails, of which 9.2 billion (73.2%) were identified as spam. Their anti-virus system scanned 147 billion emails in 2004, and 901 million (6.1%) contained viruses.
They’ve got another spam report worth reading, with lots of great quotes from various studies on the "damage that spam does to profitability". They estimate that in 2003, spam cost businesses in the US $10billion.