"A Mac isn’t susceptible to the thousands of viruses plaguing Windows-based computers."

Pretty clear.

It's not at all clear. It's deliberately ambiguous. If they wanted it to be clear they would have said "Macs are only susceptible to a fraction of the number of viruses plaguing Windows-based computers."

Clear? One possible interpretation of just that statement alone could easily give the impression that viruses in general are therefore not an issue for Macs.

There are two possible ways to interpret this:

1. "A Mac is susceptible to viruses. But it is not susceptible to viruses plaguing Windows-based computers."

2. "A Mac isn’t susceptible to viruses, whereas a Windows-based computer is susceptible to thousands of viruses."

The fact that you still get viruses, but they just don't happen to be the same viruses, isn't worth stating. So as a customer, it is very unlikely that I would infer the former (Meaning 1) from the statement. Yet it is what is meant.

I would personally call this misleading (and dangerously close to lying).

No, it's not clear. It implies that the Mac can't get any viruses.

No. If they didn't qualify it with "PC" it would have that implication. By qualifying it, they make it unambiguous.

They're implying that only PCs have viruses.

No, they are stating that Macs are not susceptible to PC viruses, which is true.

They are implying that viruses are a severe problem on PCs that Mac users do not face. This was undoubtedly true when they were running those adverts.

Yes. They are stating that Macs are not susceptible to PC viruses. Yes, this is true. However this is not worth stating, so most people would infer that Macs cannot get any viruses from that sentence.

It may not be worth stating to people round here who are essentially experts in the field, but to the vast majority of everyday PC users it's not at all obvious and very much worth stating.

According to the paragraph that follows: A Mac isn't susceptible to the thousands of viruses plaguing Windows-based computers. That's thanks to built-in defenses in Mac OS X that keep you safe, without any work on your part.

They're saying PCs get viruses and Macs don't because of the way it's designed. Not that PCs get PC viruses and Macs get Mac viruses.

They are also implying that it's the sole reason for the lack of viruses, when it's mainly the lack of users. That would explain why it's not susceptible to every other type of software available on Windows.

Next sentence: "That’s thanks to built-in defenses in Mac OS X that keep you safe, without any work on your part."

I guess they should be "thanking" the general incompatibility with Windows binaries.