Boot Sector A boot sector virus, as the name implies, infects a disk's boot sector and replaces the sector contents. This infection will cause the virus to be loaded into memory before anything else. The virus is usually passed by floppy disks. Some boot sector viruses can spread over a network connection.

File Infector The virus infects an executable (program) file. When the file is executed, the virus loads into memory and begins infecting any other executable files it can find.

Macro  This is a type of infection virus, in which the virus is based on application macros. The virus is often specific to one type of data file or application, but will infect all files of that type that it can find.

Trojan A Trojan (also called a "Trojan horse") falls into the general category of potentially destructive programs, but is technically not a virus. A Trojan appears to be a valid program, but demonstrates its destructive nature when executed. A Trojan can be, for example, an illicit server that defeats an operating system's native security mechanisms.

Email Most e-mail viruses are macro viruses, often specific to one application or one mail system. Some are Trojan programs attached to the mail message and attacking the system when opened. Worm A worm is technically not a virus. It can behave like a virus, except that a worm can spread without any user intervention. Often sent through e-mail, but can actually exploit various protocols, including HTTP, FTP, and SMB.

TSR A virus that executes, then resides in memory. TSRs will then proceed to consume system memory and interfere with any process loaded into system memory. TSR viruses are generally loaded via the Windows registry or other files at boot time, and will continue to interfere with normal system functions. They will continue to replicate to other systems until eliminated.

                                                         BACK to Computer Viruses