Malware is everywhere
The bigger the market share the bigger the target, and that means Android is ripe for malware. Add an app store without much vetting and you have a bad situation on your hands.
There are a lot of antivirus apps available for Android, and like everything in the Google Play store, caution is required. Antivirus apps need lots of access to your phone, so make sure you completely trust whatever you choose to install.
If you're wondering which apps you can trust take a look at these five and some of the features that make them unique.