[from this article in NYTimes — login required]
Spam is back … Worldwide spam volumes have doubled from last year … unsolicited junk mail now accounts for more than 9 of every 10 e-mail messages … Spammers [succeed] by conscripting vast networks of computers belonging to users who unknowingly downloaded viruses and other rogue programs. The infected computers begin sending out spam without the knowledge of their owners … 250,000 new computers are captured and added to these spam “botnets” each day.
(Hysterical but not unreasonable emphasis mine)