The Windows operating system uses the Autorun.inf file found in removable drives (CD/DVD or USB) to know what to do when they connect to the PC. Autorun.inf is a configuration file normally located in the removable drive root directory. It generally includes a reference to the removable drive icon, a description of the content and the possibility to specify the program to be run on connecting the drive.
The problem lies in the fact that there are types of malware that exploit this feature to spread to computers as soon as the USB device connects to them. The malware copies a malicious executable file to the drive and modifies the Autorun.inf files so that Windows runs it when the drive is connected. An example of this is the Conficker worm, which spreads through the Internet and also using USB devices.
By vaccinating the computer you completely disable the AutoPlay feature, so that no program on any USB/CD/DVD drive will be automatically run, regardless of whether those drives or devices have been previously vaccinated.
When vaccinating USB drives, the vaccine permanently blocks the Autorun.inf file to prevent malware from spreading automatically. When the vaccine is applied to a USB drive, the Autorun.inf file is blocked permanently, preventing it from being read, deleted or modified. Once the vaccine has been applied, Windows cannot automatically run any malware that might be on the USB device. After vaccination, the USB drive will continue to work properly, but no multimedia files it might contain will run automatically.
Bear in mind that the vaccination of USB devices is irreversible. In certain improbable situations, some applications might cease to operate correctly. In any event, if your USB device does not contain the Autorun.inf file, it is perfectly safe to vaccinate it.