Anti virus software is not needed for android (or ios for that matter). The reason you "haven't got a virus yet" is because there aren't any. There is however from time to time malicious applications on the market that you should watch out for. Any app that asks for to many permissions or permissions that don't seem appropriate should be avoided. Google does a really good job of keeping those type of apps out of the market, but other "app stores" aren't always on top of it. Google "anti virus on android, droid forms" for a few really good explanations why anti virus software isn't needed. Also, it uses up system resources and can slow down the phone.
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