After all, just one virus on a computer is one too many.
Worrying is like a rocking chair, it gives you something to do, but it gets you nowhere.
As every new breed of virus is conceived, created and released into the wild, another small change is made to the anti-virus software to combat the new threat.
There is one major problem with anti-virus software: It needs updating. Users cannot be relied upon to have even the anti-virus software in the first place, let alone be able or willing to pay for the updates.