Most viruses are written with a malicious intent, so that they can cause damage to programs and data in addition to spreading themselves. Viruses infect existing programs to alter the behavior of programs, actively destroy data, and perform actions on storage devices that render their stored data inaccessible.

Computer viruses attack the software of a computer such as operating systems, data files, application software, and e-mails. However, viruses do not affect the computer hardware.

Viruses infect computers and spread themselves using various methods. Some viruses attach themselves to a common program such as a popular game or word processor. When a person downloads the infected game and runs it, the attached virus program gets executed. The virus then loads itself into memory and searches for any other program on the disk. If the virus finds any program, it modifies the program by adding the malicious code to the program. The next time this program gets executed, it infects other programs, and the cycle continues thereafter.

There are many types of computer viruses such as a boot-sector virus, macro virus, e-mail virus, etc.

A boot-sector virus infects the boot record on hard disks and floppy disks, which is used to start the computer. When the computer is turned on or restarted, the virus is automatically executed. An infected boot disk may stop the computer from starting up.

A macro virus is a macro or script that attaches itself to a file or template. When the file is loaded, the instructions of the macro or script are executed.

An e-mail virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to many people in the their e-mail address book.

Some other types of programs are also harmful to computers, which are not viruses such as a Trojan horse.

The main difference between a virus and a Trojan horse program is that a Trojan horse program does not replicate itself. A Trojan horse only destroys information on the hard disk. A Trojan horse program disguises itself as a legitimate program such as a game or utility. It often looks and initially acts in the same way as a legitimate program, but when it is executed, it destroys or corrupts data. A Trojan horse program can contain viruses, but it is not a virus itself.

A Worm uses computer networks and security holes to replicate itself. A Worm scans the network for another computer that has a specific security hole. It copies itself to the new computer using the security hole, and then starts replicating from there as well. A virus can have both virus and worm characteristics.

There are several anti-virus software that can detect, identify, and remove these viruses. As a precaution against viruses, we should take the following actions:

An anti-virus software that can detect, identify, and remove viruses should be installed on the computer.
All information downloaded from the Internet or some unknown resource must be immediately scanned for viruses by using an anti-virus software.
Any e-mail attachment that comes from unknown sources should never be opened.
 
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