The vast majority of apps in the official app stores (Google Play and Apple) are generally safe. The most common way mobile devices get infected is by "side-loading" apps from non-official app stores.
Side-loading involves transferring an app to your phone via USB, Bluetooth or WiFi. These apps are typically advertised as free versions of premium/paid apps.
Always download apps directly from the official app stores, confirm the publisher, and check the reviews.
Metadefender - Mobile Security will be adding an virus / malware scanner very soon!
This article was last updated on 2015-08-02
(This article was changed to reflect the new name of Metadefender - Mobile Security on 2016-02-24)