You don't necessarily need to visit disreputable sites only to get these infections.
Many web pages may get hacked , and can simply radiate an attachment as you land on them!
Facebook and some of the other social networks can also get hacked, and of course the P2P downloaders are a good source of unfriendlies at times.
A Good protective help is to install PeerBlock, as it monitors all the open ports ...
- Incase you don't realise when you connect to the internet you actually have 65,000 ports open, and most of us will only use about 5 or 6 of these commonly for browsing and email - the rest are sitting there with an Open Window to the World !
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