Nothing beats avast anti virus for android, especially if you're a root user.
Others to look at are bit defender, Norton, Zoner and lookout but of the all avast takes the cake.
It can survive factory reset and has all sorts of anti theft commands that can be accesses from web or sms. A built in network meter allows you to moniter data use on WiFi and cellular. You can also control application permissions so they no longer get to view contacts or cell number etc.
There is only one thing it doesn't do, something Norton doesn't do either and that is scan an app before install it only scans once installed. also its 100% free no pay to unlock features.
If your phone isn't rooted its recommended because you'll be able to get a PC like firewall with avast as well as all the other rooted features which make it such a powerful and invaluable app.
+1 for avast.
However, if you don't just download retarded crap left and right and view porn constantly, you won't get a virus. I only have avast on my phone for the "locate my phone" feature it has in case it gets stolen. You really can't get a virus on an android unless you're an idiot.