If you download a lot of apps from unknow source (places other than Google’s play store), chances are that your phone is already infected with Malware. Trend Micro reports that there are over 500 sources that host malicious apps. While most of Malware will send texts to premium numbers without your knowledge, some could also be mining your data.
If you must get the apps from unknown sources you should get an anti-virus app. Some of the best free options include Avast, AVG, Lookout, Norton, ESET and Zoner. Note that you shouldn’t install more than one of the seats one time and having one isn’t a guarantee against bad apps.