You thought you saved humanity already? "On the Brink: Mutation" DLC brings a fifth, special, highly dangerous and purple, disease to eradicate.During infections, the Mutation will pop up unpredictably and becomes a new disease to fight in contaminated cities. Making the game more challenging than ever before, this disease divides the team’s efforts and will cost them precious time.You’ll need to plan ahead and coordinate the whole team to fight the new virus and find a cure for the 5th disease.

Go to: http://store.steampowered.com/ and download STEAM client Click "Install Steam" (from the upper right corner) Install and start application, login with your Account name and Password (create one if you don't have). Please follow these instructions to activate a new retail purchase on Steam: Launch Steam and log into your Steam account. Click the Games Menu. Choose Activate a Product on Steam... Follow the onscreen instructions to complete the process. After successful code verification go to the "LIBRARY" tab and start downloading.