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 is that there is malware that exploits this feature to spread to other computers and infect them as soon as a USB drive is connected. The malware copies a malicious executable file to the drive and modifies the file Autorun.inf so that Windows runs it when connecting the drive. 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.
By vaccinating a USB device, the vaccine blocks file Autorun.inf permanently to stop malware from spreading automatically. When a vaccine is applied to a USB drive, the file Autorun.inf is permanently blocked, preventing it from being read, deleted or modified. Once the vaccine has been satisfactorily applied, it will prevent Windows from automatically running any malware found on a USB drive. After vaccination, the USB drive will continue to work properly, but no multimedia files it might contain will run automatically.
It is important to bear in mind that USB drive vaccination is an irreversible process. Under extremely unlikely conditions, certain applications might stop working correctly. In any event, if your USB device doesn't contain the file Autorun.inf, it is completely safe to vaccinate it.