I remember back when I was in vocational school and my computer was an Intel 486SX (no floating-point co-processor!) with 8 MB of RAM (at a time when 4 was usual) running Windows 3.1.
Back then, I used to laugh occasionally at Amiga users who could get infected by a virus simply by inserting a diskette into their machine; on DOS, you'd have to boot from the floppy or manually start a program.
And where have we arrived now? Stick a USB stick or a CD in the machine and, by default, a program will be started automatically if the medium has certain files and settings on it.
So much for that line of defense.
(I also remember saying that it's impossible to get infected by a virus merely by opening an email message. Now, with HTML email and other active content being common, see how true that is.)
Ah, sic transit gloria mundi (Gloria threw up on the bus on Monday).