In ADDITION to the Apple software updates (this latest Java-protection one is hefty, weighing in as I said at about 75 meg) I also have two free antivirus programmes, both of which I like very much. They are totally non-intrusive and update fast with no fuss. The first one below does this automatically, and the second needs to be clicked, but it's instant. They are:

iAntiVirus from PC Tools
http://www.iantivirus.com

VirusBarrier Express from Intego

This is downloadble through the "App Store" pre-installed on your Mac, so open it and just type "VirusBarrier Express" into the searchbox and it will lead you there. There also is a VirusBarrier Plus which you have to pay for (not expensive) but I only have the free one. I saw a negative comment about it in the reviews but please note that this was from a few years ago and referred to a previous version. Certainly I have been very happy with mine. You do have to remember to click its little update bar, but I keep a shortcut icon on my desktop and this is literally a matter of seconds.

Both of these apps are free and no trouble at all, so I strongly recommend them. The VB also has a little portal in which you can drag an individual item to scan if you don't like the look of something.

Welcome to the world of popular success, Mac. Now YOU TOO are important enough to get picked on. *sigh*