A quick guide to computer viruses - what they are, how they work and the potential consequences of a virus infecting your computer

A computer virus is a harmful program that is designed to insert unwanted code into another program or file (the host file).

Each time the host file is run, the virus code in it will also run and insert more unwanted code, either into the same file or into other files on the same machine. If this happens enough times, the unwanted code can seriously disrupt the infected program's normal use, or corrupt it entirely.

Some of the most notorious computer viruses in history include:

Infecting files

Viruses can be distributed and installed on a computer in many ways, though the most common methods usually involve either:

  • Social engineering to trick the user into receiving and installing the virus themselves;
  • Exploiting a vulnerability to silently install the malware;

Or some combination of the two.

Once the virus arrives on a machine and is run, it begins its attack on the files in the system.

Viruses can infect many different types of files or programs - critical programs used by the operating system; document files such as Word or Excel; even specialized firmware programs tied to the computer's hardware, like the Master Boot Record (MBR).

For this reason, viruses are often grouped by the type of file they infect, such as boot viruses and macro viruses.

Damaging the infected files

Each time a host file is run, the virus code in it will replicate - that is, it will create and insert more unwanted code, either into the same file or into another file on the same machine (essentially infecting the other file as well).

As this process repeats, the increasing additions of unwanted code can disrupt the host file's normal operations. If it happens often enough, the virus code can completely corrupt the host file.

In addition to infecting its host files, a virus can often perform other harmful actions on the affected computer. These actions can range from simple nuisances to severely harmful:

  • Changing the desktop background
  • Playing sounds or displaying images
  • Deleting files and programs
  • Modifying or stealing sensitive data files

Depending on what other actions the virus performs, an infection can be devastating.

Viruses range from fairly simple programs to very sophisticated constructions. Some include features that are similar to the functionality of trojans or worms; others are capable of constantly changing their own code to avoid detection by antivirus programs.

These additional abilities can make it more difficult for users to identify and counter any problems caused by the virus. This wide variety of capabilities are best seen in the following viruses:

The decline of viruses

Viruses used to be the most common type of harmful program that users encountered in the 1990s; today, most people are more likely to encounter trojansworms or other types of threats.

Though the total number of actual virus infections have dropped precipitously over the years, they still remain a threat to users using older, unprotected operating systems or programs.