A virus is a program that will seek to duplicate itself in memory and on disks, but in a subtle way that will not immediately be noticed. A computer on the same network as an infected computer or that uses an infected disk (even a floppy) or that downloads and runs an infected program can itself become infected. A virus can only spread to computers of the same platform. For example, on a network consisting of a WinTel box, a Mac, and a Linux box, if one machine acquires a virus the other two will probably still be safe. Note also that different platforms have different general levels of resistance; UNIX machines are almost immune, Win '95 / '98 / ME / XP is quite vulnerable, and most others lie somewhere in between.