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A computer virus is an illegal and potentially damaging computer program designed to infect other software by attaching itself to any software it contacts. In many cases, virus programs are designed to damage computer systems maliciously by destroying or corrupting data. If the infected software is transferred to or accessed by another computer system, the virus spreads to the other system. Viruses have become a serious problem in recent years, and currently, thousands of known virus programs exist. Viruses can be categorized as boot sector viruses, file viruses, and Trojan horse viruses. A boot sector virus infects the boot program used to start the system. When the infected boot program executes, the virus is loaded into the computer's memory. Once a virus is in the memory, it can spread to any floppy disk inserted into the computer. The second type of virus, a file virus, inserts viral code into program files. The virus then spreads to any program that accesses the infected file. A Trojan horse virus (named after a Greek myth) hides within or is designed to look like a legitimate program. Some viruses interrupt processing by freezing a computer system temporarily and then displaying sounds or messages. Other viruses contain time bombs or logic bombs. A time bomb is a program that performs an activity on a particular date. A logic bomb is a program that performs an activity when a certain action occurs, such as an employee being terminated. A worm, which is similar to a virus, copies itself repeatedly until no memory or disc space remains. To detect computer viruses, anti-virus programs have been developed. Besides detecting viruses, anti-virus programs also have utilities to remove or repair infected programs and files. Some damaged files cannot be repaired and must be replaced with uninfected backup files. Computers can best be protected by installing a virus protection software. In order to be effective, it is imperative to develop a regular plan for copying and storing important data and program files. Prior to running the virus protection software, floppy disks should be scanned using a virus scan. Since one of the most vulnerable points of entry for viruses is the internet, all programs and email messages downloaded from the Internet should be checked to make sure they are virus free.