A virus is the earliest malicious code to weaken Internet security. A virus does not spread to other systems on its own but it can self-replicate. An infected file must be sent to another system, so its spreading options are limited. A virus resides in the systems memory and reproduces its own code while the program to which it is attached is running. Some viruses do not reside on the systems memory after the execution of the program to which it is attached is complete, and it leaves the memory once the host program is closed. However, viruses that leave the memory, like other viruses, have already attached themselves to files such as .com, .exe, or .sys. Viruses can infect the boot sector, programs, the network, source code, files, macros, and data files. Elk Cloner was the first virus written specifically for a Mac OSx. It was particularly designed for Apple DOS 3.3. Ever since, every infected Mac OSx has had some form of this virus. In 2010, the Stuxnet virus was created for cyberwarfare against the Iranian government. The public morphed it to take advantage of four zero-day exploits in the Microsoft operating system. The threat was so serious that the US-Cert issued an Advisory, ISCA-10-272-0, on how to detect Stuxnet (ICSA-10-272-01- Primary Stuxnet Indicators, 2010).