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Well like it or not the numbers game will come into play. The worm/virus would not spread very quickly just because small % of computers running OSX.
The virus is going to attack more or less randomly targeting computers. Only problem is only like 5% of the shots will hit a computer that is able to be attacked by the virus. 95% of the computers out there would not spread the virus. The security though obscenity is not a myth. It a is a fact.
It is a fact not because there are few computers out there so few virus are made for it (it helps) but because it takes a while for something like that to spread.

The Mac users in general I would classify as slightly better than the average PC users in how well they keep informed about computers and that is still pretty much 90+% of them have no clue what is going on and I willing to be a vast majority of them there computers are not up to date.

Add to it the complete lack of antivirus programs on OSX computers adds to the problem if a virus did hit. It be slow to spread but when a cure for it comes out it would be even slower to get out because vast majority of the Mac computer would not have an Antivirus program on there computer. Plus those same computers with out the antivirus program would be easier to hit because they have no software to look at things that come in for virus like tendencies in it.
 
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