Dont just take our word for it...
Below are listed just a few of the thousands of articles posted onto the web about computer viruses. Note that they all have one thing in common, they were all contracted on computers running a Microsoft operating system:
 

Double attack fires 650,000 trojan emails at U.K. firms 22-03-2006 by William Eazel

Computer virus fells Russian stock exchange 03-02-2006 by Will Knight

Nigritude online virus hits over 150,000 servers 20-05-2004 by Bird, Day & Loft

Navy investigates US Navy Marine Corps intranet's Welchia outbreak 29-08-2003 by Matthew French

Anna virus the work of 'script kiddies' 13-02-2001 by James Middleton

Killer virus unleashed today 13-02-2001 by Robert Jaques and John Geralds

Virus Attacks! 12-02-2009 by Dan Goodwin
 

...And some interesting True Virus Horror Stories can be found here