Windows is like a bad neighborhood, strewn with litter, mysterious odors, panhandlers, and untold dozens of petty annoyances. The Mac is like a good neighborhood, where the streets are clean and the crime rate low. You don’t need bars on your windows in a good neighborhood; you don’t need anti-virus software on the Mac.
The security disparity between the Mac and Windows isn’t so much about technical possibilities as it is about what people will tolerate. And Mac users don’t tolerate shit.
The consistently eloquent technology writer John Gruber describes the reason why Windows is ridden with spyware, adware, and viruses.