Originally posted by Riek
How do you actually know this?

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If you do not have AV installed, it is only a matter of time before you are infected.
Yes, but conversely, how do you know that having AV installed makes you safe? Antivirus (etc) programs are like any others... they have their limitations. Usually they get updated only when something's discovered, which means that if you're one of the unlucky few to be in that first batch of infections, you're still in trouble. It's not immunity, only protection against what's already known.