Virus / Malware / Adware
It sneaks onto your phone stays in stealth mode until you activate it and starts stealing information from then.
1.Sudden Increase in Data Usage
If you could see sudden spike in monthly data usage that could be due to the fact that the phone might transfer your data remotely without your knowledge.
2.Sudden App Crash
Sudden frequent App crash time to time could indicate the presence of virus in the device.
Most of the annoying apps you see as a popup contain adware spammed program however you can bypass this by using a Ad Blocker or turning on the popup blocker in the browser setting.
4.Quick Battery Drain
If your phone starts draining battery faster virus program might have took over the system and inturn enables lot of background apps causing system overload and fast battery discharge.
5.Battery Backup :
If your battery drains on the midday even with a full charge in the morning.The phone’s battery life gets reduced every time when the power goes 10% to 100%. If your phone’s battery dies suddenly at random time then it might be the time to replace the battery.