The message: "You have (5) viruses on your phone. Click 'Ok' to download a free app to remove it.", is a fake. The message is spam designed to trick you into buying software, by attempting to frighten you into believing that there are viruses on your mobile device or computer. If this message is not displayed by your antivirus software, you can ignore it. Only your antivirus is able to determine if your mobile device or computer is infected.
Scammers distribute these spam messages via online advertisements.
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This spam message is similar to the following: