sure nothing is 100%.
But fourteen years on macs now.
Not once had a virus, malware or a security breach - and I don't run anti virus software.
It isn't about ubiquity of market, 16% market share in the US (and if that isnt a substantial enough market of people doing internet banking, credit card details etc) according to recent reports. It's just a safer platform. Things change and that may not always be the case, but for now its a reality.
Barring the thing above which sounds like it works by getting the user to download and install it. No computer is safe if an idiot is controlling it.