A computer virus is not a biological agent which infects your PC, it’s simply a computer program. When you buy software from a store, you are purchasing a program which was designed or written to perform a specific job you want to have done on your PC. The software can be a game, a word processor program like Microsoft Word or a CD label maker program. Whatever it is, all programs tell your computer what to do and how to do it. A computer virus is also a program which tells your computer what to do when it is run. The term “virus” how­ever, is what identifies a specific type of program. One which has the ability to self-replicate and to travel which other programs do not do. Self-replication is what gives a virus program a parasitic nature which is neither an accident, nor a computer glitch. Viruses are written by programmers who know what they are doing and have designed virus programs to do their bidding.

As stated, a computer program is termed a “virus” when the program reproduces by making a copy of itself. The copy is then attached to another computer program or data file which allows it to propagate as the file is copied and sent to other PCs and opened. A program is termed a “worm” when the method of reproduction is accomplished by simply making a copy of itself and is not attached to anything. These are the most destructive as they run in the background and continually make copies and transmit the copies through email and local networks. Worms are well known for replacing operating system programs with themselves in order to keep the worm running. Malware is short for malicious software which typically enters by visiting the wrong web site. Malware is specifically designed to damage or disrupt a system through the use of internet Java programming.

Viruses are typically first detected by large anti-virus companies who write programs which protect your PC from viruses, worms and Trojans. These companies are those who also are the first to designate a unique name to each new virus found. Sometimes different anti-virus companies give different names to the same virus which can make things more confusing. Each company keeps a master list of all their known viruses along with information on what it does, how it propagates and how to remove it. These lists are typically published and available on the web site of each anti-virus company.

 How did computer viruses come to be? The first theories about the possibility of someone creating a self-replicating program dates as far back as 1949. Experimental viruses were first written and tested in the early 1960s to study their ability to move throughout networks. They received their official name in 1984 when a university professor used the term "virus" comparing them to a biological virus. Like a biological virus, a computer virus is small, makes copies of itself, and cannot exist without a host. When personal computers became more popular around 1986, more and more viruses began to appear. At first they were intended as jokes, or developed for research or demonstration purposes. It didn’t take very long before they became a lot more destructive.

Here is an example of how fast a virus can spread. The fastest spreading virus to date was known as the "Slammer Worm" It began infecting hosts just before 5:30 UTC on Saturday January 25, 2002. The virus exploited a Microsoft server vulnerability to enter. Once in the server, the Slammer generated a set of random Internet addresses and scanned them for other unprotected computers which it then also infected. From the computers it infected, it generated more IP addresses, scanned them and infected further. The Slammer worm was relentless. In the first minute, it was doubling the number of infected computers every 8.5 seconds. In just three minutes it was scanning 55 million targets per second. Within 10-minutes, over 90 per­cent of all unprotected computers in the world were infected. Even though the worm did not contain a malicious payload, it caused considerable harm simply by overloading networks and taking database servers out of operation. This resulted in unforeseen consequences such as canceled airline flights, interference with elections and ATM failures.