If you access the Internet or receive email, you're computer will be subject to electronic infection. Left unchecked and uncorrected the damage these electronic infections inflict can be devastating. But what are these electronic infections? What can they do and how are they propagated? The three main categories of electronic infections are viruses, worms and Trojan horses.
A virus is a small piece of software that piggybacks onto real programs. For example, a virus might attach itself to a spreadsheet program. Each time the program runs, the virus reproduces itself (by attaching to other programs) or wreaks havoc directly. Once it is running, it can infect and spread to other programs or documents.
Most viruses also have some sort of destructive attack phase. A trigger will activate this phase and the virus will "do something," ranging from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date or the number of times the virus has been replicated.
A worm is a computer program that has the ability to copy itself from machine to machine through computer networks. To infect a machine, a worm usually exploits some sort of security hole in software or the operating system. Each time a worm finds an unsecured server or PC, the worm will copy itself to that server or PC. The new copy then scans for other servers or PCs to infect. Depending on the number of unsecured servers, a worm could conceivably create hundreds of thousands or even millions of copies in very little time.
Worms cause damage by using up computer time and network bandwidth when they are replicating. One such worm was called the Slammer worm. It essentially ground the entire Internet to a halt by sending out millions upon millions of packets of meaningless information which jammed critical Internet servers causing corporate email systems to crash and various web sites to become unresponsive. Other times a worm may replace web pages or generate excess traffic to a particular web site in order to overwhelm it.
A Trojan horse is a computer program, which, as the name implies, claims to do one thing such as a game, but instead does damage when you run it (i.e. erasing your hard disk). Trojan horses are almost always designed to do harmful things, such as erasing or overwriting data, corrupting files in a subtle way, setting up networks of zombie computers in order to launch DDoS attacks or send spam, spying on the user of a computer, logging keystrokes to steal passwords and credit card numbers, "phishing" for bank or other account details, or installing a backdoor on a computer system to enable unauthorized access by hackers. Trojan horses get around through email, web sites, other infected programs, or through open ports on the network.