Personally I don't use an Anti-Virus app on my devices, personally I don't think they work on Android as they do on a PC.
There are three main types of malware floating around at the moment, the annoying but not so dangerous ad push malware and the premium rate phone number apps which as Wi-Fi only tablet owners don't have to worry about.
There are also malware apps that can root your device but as mentioned above, you'd have to install that in the first place. There's been a recent case of Android tablet/phones that are infected whilst browsing the internet, coming across a page that claims the content cannot be viewed without the installation of an application, whether download authorization is required or not the user would have to confirm installation of this downloaded app...
Another recent attack has been due to the lack of a Flash on Android 4.1 or above, a Google search will reap many hits while looking for a Flash apk but some of those could be malware and most of those are supposedly Russian in origin but nonetheless, the lack of Flash support on 4.1 devices has left an opening for unscrupulous people.
A user on here recently posted how he/she was using one of the big name Anti-Virus apps and installed a battery optimizer application, this supposed optimization application was in fact an ad push application, therefore ads were pushed to the notification screen...
A friend had the same thing, some of those ads were most certainly not child friendly...
If a big name Anti-Virus application cannot detect an ad push application, what else can it not detect?
My personal opinion is as fxsarge stated, be mindful of what you are installing. Unknown sources is there for a reason, it's a security feature to protect you installing dubious apps from outside Google Play. Yes malware apps are found on Google Play but rarely and Google does act pretty quickly when they do appear.
Bottom line is, be aware of an apps permissions, if you don't think an application needs the permissions it is asking for then don't install it.
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