A virus is a program that can “infect” other programs by modifying them. Modification includes a copy of the virus program, which may infect other programs. Computer virus has similarity with biological virus, a biological virus infects the machinery responsible for the living cell to work and a computer virus carries in its instructional code the recipe for making perfect copies of it.
The typical virus takes temporary control of the computer`s disk operating system. Then, whenever the infected computer comes into contact with an uninfected piece of software, a fresh copy of the virus passes into the new program. Thus, the infection can be spread from computer to computer by unsuspecting users who either swap disks or send programs to one another over a network. In a network environment the ability to access applications and system services on other computers provides a perfect way to replicate itself.
Network worm programs use network connections to spread from system to system. Once active within a system, a network worm can behave like a computer virus, bacterium, It can implant Trojan horse programs or perform any number of disruptive or destructive actions. To replicate itself, a network uses some sort of network vehicle. Some of its uses are Electronic mail facility, Remote execution capability, and remote login capability. A worm mails a copy of itself to other systems via electronic mail facility. A worm executes a copy of itself on to a remote system as a user and then uses commands to copy itself from one system to the other.
Then new copy of the program is then run on the remote system where, in addition to any functions that it performs at that system, it continues to spread in the same fashion. A network worm exhibits the same characteristics as a computer virus. A dormant phase, a propagation phase, a triggering phase, and an execution phase. The propagation phase generally performs the following functions, Searches for other systems to infect by examining host tables or similar repositories of remote system addresses. It establishes a connection with a remote system. Copies itself to the remote system and causes the copy to be run.
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The network worm may also attempt to determine whether a system has previously been infected before copying itself to the system. It may name itself as a system process or using some other name that may not be noticed by Adminsitrator.