Over-analyzing ad wording...
) may notice that this claim could be interpreted as any of these:
Logicians and nitpicking grammarians (like me, for examplethe world's most advanced operating system that doesn't suffer from all the viruses that plague PCs
"Macs suffer no viruses." (a very strong claim)
"Macs suffer no PC viruses." (but might have their own)
"It is not the case that Macs suffer from every single PC virus." (a very weak claim)
"Of all the operating systems, this is the one that suffers least from viruses." (i.e., the virus problem might be awful under Mac OS X, just not as bad as for Windows)
To avoid that last interpreation, they could have changed "that" to "which" and added a comma, so it would be"Macs suffer no PC viruses." (but might have their own)
"It is not the case that Macs suffer from every single PC virus." (a very weak claim)
"Of all the operating systems, this is the one that suffers least from viruses." (i.e., the virus problem might be awful under Mac OS X, just not as bad as for Windows)
the world's most advanced operating system, which doesn't suffer from all the viruses that plague PCs
That change would also avoid the interpretation that Windows is more advanced and Mac OS X is merely the most advanced O.S. within the class of virus-free operating systems.