Originally Posted by David Munch
It is not possible for a file to wreak havoc on its own in iOS, since all apps have to be signed from Apple, massively lowering the chances of something going wrong.
I wouldn't go as far as not possible, it's unlikely, but still possible. For example the iOS PDF exploit where just viewing a PDF on the web could root your device and take full control. Patched, but for a while was a threat.
What you usually won't find, is popping any old executable on the device and running it to "infect" it. Since everything has to be signed (the one app that allowed running of unsigned code and was pulled not withstanding).
There are absolutely no virus' available for OSX, and as long as you use sensible approaches to downloading things from the shady sides of the internet, ie. not installing trojans (And to that extent - Any app that requires your admin password for installation, which doesn't belong to a trusted source, is a no no), there is absolutely no way your machine will get harmed.
If by virus you stick rigidly to the original definition of the word, then it's likely true. But if you expand that to include malware such as trojans or worms, then it's not. There's less of it, certainly, but that is likely just down to user base.
The malware that targets the Mac tends to be trojans or worms (which is where windows malware is moving to as well). There's an argument for installing virus software if the virus firms are updating their definitions quick enough when new trojans are found. However, on any OS, trojans are a bit of a losing battle as you can only be protected against the old/known ones.
You're reasonably secure as long as you stick to "safe" sites. However, not absolutely secure. See the flashback issue that used a java exploit to nestle malware into the user account and in some cases also gained root access. With XSS, even "safe" sites can be used to hit your machine.
Had users been running scanners, then once the "exploit" was known about, those scanners may have been updated in time to prevent additional users getting infected. Whilst Apple issued a patch quickly when it became public knowledge that the exploit was been used by criminals, they had dragged their feet for a few months not pushing a java security fix Sun had issued.
I don't think Mac users have all that much to worry about when it comes to Virus', however, Virus' are no longer the norm for targetting machines, it's worms and trojans. Both of which can impact the mac just as readily as they can windows, as in both cases they focus on either user flaws or OS/app exploits. Which both platforms have in ready supply.
Gatekeeper should help to an extent. Although windows has had something similar for a while and devs/users have been slow to adopt it. Maybe Apple will be able to push adoption of it better than MS managed.