Your oldest enemy: Virus

Virus is a piece of malicious software that modifies the original executable file by appending portions of malicious code to its contents. It then replicates by modifying other executable files on a system, including those stored on local disks and removable storage devices. The goal of a virus is to infect as many adjacent binary files as possible, while carrying no malicious functionality beyond propagation. The side effect of virus contamination sometimes renders the original file being corrupted and no longer usable. Example of a virus is Mrak5 which contaminates exe-type programs.