Antivirus software is all about prevention. It is used to prevent files that contain viruses from being downloaded onto your computer. It also tries hard to prevent the virus from being activated, should it somehow get downloaded to your computer, placed in memory or in a file-like location. If the virus-laden file is never downloaded, no problem. And if the file is downloaded, but flagged by antivirus software as malware and prevented from being activated, it won't cause any damage to your system — though the infected file still needs to be contained and deleted.