Virus is a new mode that is featured in Bloons TD EXTREME.
Virus is a mode where all of your towers get a virus. The virus slows them down by 5%, Makes their attacks weaker (-1 popping power), and towers get sold after they have been placed for 7 turns for 50% of what you bought it for. If you make it to Round 85, you get $700 Monkey Money. This track is difficult.