Simply install the apk on someone's phone and voila! The person wont be able to use his phone. A blank screen with the words "VIRUS!!!" will be presented. The only option for the person to use his/her phone again would be to restart his phone (force shutdown or removing battery).
But if you (or the person) thought that, that was it then you are mistaken. After restarting the phone again the black screen is presented to the user.
P.S. This app was intented for fun purposes :) Hence, a workaround to remove the black screen is to long press on the screen for more than 5 sec and you will be able to use the phone as before.