How to detect a virus on your Android Phone?

So what is malware?  
Malware, short for malicious software, is software designed to secretly control a device, steal private information or money from the device's owner. Or some malware has been used to show advertisement and lure you to click.
  How to judge?  
Surge in data usage
One of the first things to check is your monthly data usage.  If you notice sudden spikes in your data usage even though you haven't really changed your usage patterns, then chances are you are infected. Adware infected phones usually perform unsolicited clicks in the background to generate profit for cybercriminals. 
Sudden pop-ups
If you're starting to get annoying pop-up ads and notifications, unwanted reminders and nagging "system" warnings that just won't go away, then your Android phone may have been compromised. Apart from slowing down your phone and eating away at your data, these intrusive notifications can also install more malware on your phone.
Unwanted apps
Keep an eye out for apps that you don't remember installing. Trojan malware, especially the adware, are known for automatically downloading further malicious apps without your knowledge. 
  Steps you can take to remove an Android virus  

Remove questionable apps
To review and remove questionable apps, go to Settings >> then Apps or Application Manager. Look through the list and keep an eye out for anything that's odd or unfamiliar. Tap the questionable app you want to get rid of and this will open up the App Info screen. First, remove the app's data cache by hitting "Clear Cache." Next, delete the app's data by tapping "Clear Data." Once these steps are done, click on the "Uninstall" button to remove the app.
  Antivirus apps  
You’d be wise to have an antivirus or a security app installed on your device. Plenty of free Android antivirus apps are available that are able to detect and remove malicious apps, for example 360 Mobile Security, Avast and Lookout. 

All Android viruses are delivered via apps installed on your device, so if your phone doesn’t already have a virus, the best way to avoid it getting one is to never install software outside of the Google Play app store.
Second, be careful with links and websites you visit. Drive-by malware downloads could happen anytime without you knowing it. Don't grant any system permissions to prompts coming from unknown sources.
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