One goal of malicious code is to propagate, meaning that it needs to find and spread to other potential hosts (systems or programs) that it can infect. Some of the more common Means of Transmission for malicious code are by the following: • Email as an attachment, using either harvested email accounts or collecting e-mail accounts from address books of infected systems. The actual sending of the e-mail can be achieved either by using existing mail server infrastructures or embedding the mail server in the payload of the malicious code. • Sharing programs infected with a Trojan horse. • Accessing Websites embedding malware. • Remaining in the computer memory and causing itself to be embedded in every program that is executed. • Infecting the boot sector of a computer’s hard disk so that the virus code is launched every time the computer is started. • Actively searching for data or programs on a computer’s storage device that the virus code can embed itself in. • Accessing shared resources such as shared file systems on file servers. • Actively using network connections to propagate (computer worms).