Computer virus
A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The term virus is commonly used to refer to many different types of malware programs.

Types
The following table shows the most common types of computer viruses and their characteristics.

Common types of computer virus

Type	Characteristic
File	Attaches or associates its code to a file. Usually, this type of plague adds the code to a file of a normal program or overwrites the file. The virus infects Windows executable files, especially the ones with .com and .exe extension types and doesn't act directly over data files. In order for its destructive power to take effect, it's necessary that the contaminated files be executed.
False Alarm	Does not cause damage to the computer, but consumes connection time and bandwidth of the internet connection to make the user send the alarm to a larger number of people. Can be classified as Hoax or Chain Letter.
Backdoor	As its name states, it's a virus that open the computer's "back door" enabling hackers to take the computer control. Normally, the backdoor virus comes embedded in files received through e-mail or downloaded from the internet. When the file is executed, the user free the virus, that opens a computer port for the virus creator so that he can gain the complete or restrict control of the computer.
Boot	Infects the initialization area of floppy discs and hard disks. Vital system files are found on these areas. This virus has a high destruction power, even blocking the user from starting the computer operating system.
Trojan Horse	Apparently inoffensive virus that brings embedded another program that is the malign virus.
Encrypted	Codified virus that complicates antivirus actions.
Hoax	Messages that usually arrive in e-mails alerting the users about a showy, highly destructive virus.
Macro	Infects macros (executable code used in text processors and spreadsheets to automate tasks). Disables essential functions as Save, Close and Exit.
Multipartite	Infects the master boot record (MBR), boot tracks and files.
Mutant	Programmed to complicate its detection by antivirus software. Is capable of altering itself after each execution of the infected file.
Polymorphic	More intelligent variation of the mutant virus. Tries to impede the antivirus action modifying its internal structure or its codification techniques.
Program	Infects only executable files, blocking the user from initializing the operating system.
Script	Programmed to execute commands without user interaction. There are two categories of the script virus: VB based on the programming language and the JS based on JavaScript. This kind of virus can come embedded in images and in files with uncommon extensions as .vbs.doc, vbs.xls or js.jpg.
Stealth	Invisible virus that uses one or more techniques to avoid its detection. Can redirect system indicators and infect a file without altering the same.

Sources
Relationship sites, torrent download sites, virtual communities as Orkut just to name a few are sources of virus. Messages or posts apparently inoffensive can have an embedded/attached virus. The virus can even be hidden in images as the one shown bellow. Take it easy. The image below doesn't have a built-in virus!

Generally, the computer virus is scattered in places of great access as is the case of pornographic sites and download networks that distribute pirate software copies (the so famous P2P networks), although nothing impedes that a virus be received in a simple e-mail message.

Harms
Most of the time the virus remains in an inactive state waiting for the user to take action. The virus can copy all the data typed by the computer user, getting a list of the visited sites, the user's preferences and even a list of all the contacts (e-mail addresses) of the user that has a virus in their computer. This forms a really dangerous virus chain in which the virus can propagate itself to other users given the e-mail addresses of the other users.

The inexperienced users don't realize the maliciousness in such appeals and end falling into the trap. They pass credit card numbers, details from where they live, where they study, what they do to have fun, etc, that is, hand over private details to a person (can be the virus creator or no) completely anonymous.

A common situation is that of an infected user that has a computer virus and not even knows about its existence till the moment that s/he receives the bank statement in which is listed the draws of tens, hundreds or thousands of dollars.

Summary
Nowadays we're witnessing an awfully race for the acquisition of new computers. Lots of inexperienced users are getting into the computer and internet world. Such users don't posses most of the time the correct information about the threats inherent to the inadequate use of computers.

Computers are excellent tools when used to legal work related tasks and healthy leisure practices,

however the inexperienced user can fall into traps when s/he decides to have fun using the computer to search for illegal tasks or practices.

With the increasingly proliferation of computer viruses it's recommended that unknown files be discarded and that visits to inappropriate sites be avoided. It's also recommend the use of an antivirus software.

In the last analysis, for those that are already familiarized with the virtual world there isn't too much danger, though the above advice must be observed. The inexperienced users must ask the experienced users for advices to gain the necessary information regarding the inherent threats present in the virtual world.