One could observe that the most effective way to mitigate a threat on the attack tree is to mitigate it as close to the "root" as possible. Although this is theoretically sound, it is not usually possible to simply mitigate a threat without other implications to the continued operation of the system. For example, the threat of viruses infecting a Windows system may be largely reduced by using a standard (non-administrator) account and NTFS instead of FAT file system so that normal users are unable to modify the operating system. Implementing this negates any way, foreseen or unforeseen, that a normal user may come to infect the operating system with a virus; however, it also requires that users switch to an administrative account to carry out administrative tasks, thus creating a different set of threats on the tree and more operational overhead. Also, users are still able to infect files to which they have write permissions, which may include files and documents.