Updated: 06/30/2020 by Computer Hope
Computer Virus

A computer virus is a malicious program, script, macro, or code designed to damage, steal personal information, modify data, send e-mail, display messages, or a combination of these actions.

When the virus is executed, it spreads by copying itself into or over data files, programs, the hard drive boot sector, or anything else writable. To help the spread, the virus, writers use knowledge of security vulnerabilities, zero-days, or social engineering to gain access to a host's computer.

Which operating systems are susceptible to viruses?

All operating systems can contract a computer virus or malware. Whether you use Android, iOS, Microsoft Windows, macOS, or a Linux variant, your computer can be at risk of getting a virus.

How does a computer get a virus?

There are many ways a computer can become infected with a computer virus or other malware. When a virus is made, it's often distributed through shareware, pirated software, e-mail, P2P programs, or other programs where users share data. Once downloaded or copied to the computer, if the infected program is executed, it can potentially affect anything that a computer can access.

Virus protection

How to detect and protect the computer from viruses

You can protect your computer from viruses by installing an antivirus protection program. Once installed on a computer, an antivirus monitors, detects, and cleans any computer viruses by looking for virus signatures.

The recommended method of detecting and cleaning the computer from any computer viruses or other malware is an antivirus or antimalware protection program.

Alternatively, a user can look at various aspects of the computer and detect possible signs indicating a virus is on the computer. While this method can locate some viruses, it doesn't determine the exact virus you may or may not have and is not recommended.

What can a virus do to a computer?

What a virus does to a computer depends on the type of virus. Most computer viruses delete data, overwrite information, display messages, and add itself to other files on the computer. Almost all computer viruses only damage the data contained on the computer and do not physically harm the computer or its hardware. For example, a non-resident virus can infect a specific file, and when accessed, it can spread to other files, corrupting those files and making them unreadable.

It is possible for more sophisticated viruses, like Stuxnet, to cause physical damage to components inside the computer or equipment controlled by a computer.

Computer virus properties

Below is a list of different computer virus properties and what each property is capable of doing. Keep in mind that not all viruses have all of these abilities and some properties are not listed.

Your computer can be infected even if files are only copied

Because some viruses are memory resident, once in memory it can infect any computer file or drive, even after the original infected program or document is closed.


They can be polymorphic

Some viruses can modify their code, which means one virus could have many different variants of itself. Polymorphic viruses can also change how they're delivered, such as changing the subject or body of the message to help keep them from being detected.

May be a stealth virus

Stealth viruses first attach themselves to other files on the computer before delivering their final payload. This type of tactic allows the virus to spread more rapidly.

Viruses can carry other viruses

Because viruses are only code, they can become infected with other viruses. Consequently, your computer can be infected with multiple viruses.

It can make the system never show outward signs

Some viruses can hide changes made, such as when a file was last modified, making the virus more difficult to detect.

It can stay on the computer even if the computer is formatted

Some viruses can infect different portions of the computer, such as the master boot record, different partitions, or other areas. Also, if a computer virus is on a backup, it can re-infect the computer when the backup is restored.

How viruses may affect a file

Viruses can affect any files; however, usually attack .com, .exe, .sys, .bin, .pdf, .pif or any data files

Viruses can infect any file, including executable files or data files, such as Microsoft Word documents or Excel spreadsheets. Most viruses target files used frequently to help spread the infection.

Increase the size of files

When infecting files, viruses increase the file size. However, with more sophisticated viruses, these changes can be hidden to make detection more difficult.

A virus can delete files as the file is run

Because most files are loaded into memory, once the program is in memory, it can delete the file used to execute the virus to hide its tracks.

It can corrupt files randomly

Some destructive viruses are designed to corrupt, lock, destroy, and delete files.

It can cause write-protect errors when executing .exe files from a write-protected disk

Viruses may need to write themselves to files that are executed; because of this, if a diskette is write-protected, you may receive a write-protection error.

It can convert .exe files to .com files

Viruses may use a separate file to run the program and rename the original file to another file extension, so the exe file is run before the com file.

It can reboot the computer when executed

Numerous computer viruses cause a computer to reboot, freeze, or perform other tasks not normally exhibited by the computer. However, keep in mind that although your computer may be exhibiting these symptoms, it does not necessarily mean your computer has a virus.

What viruses may do to a computer

Below are different issues you may experience when you are infected with a virus. However, realize many of the issues below can also be caused by another computer-related issue and not a virus.

  • Deleted files.
  • Various error messages in files or on programs.
  • Changes volume label.
  • Marks clusters as bad in the FAT.
  • Randomly overwrites sectors on the hard drive.
  • Replaces the MBR with own code.
  • Create more than one partition.
  • Attempts to access a non-existing drive, causing error messages like: "Invalid drive specification."
  • Causes cross-linked files.
  • Causes a "sector not found" error.
  • Cause the system to run slow.
  • Logical partitions created, partitions decrease in size.
  • A directory is shown as garbage.
  • The directory order may be modified to cause files such as COM files to start before EXE files.
  • Cause hardware problems such as keyboard keys not working, printer issues, modem issues, etc.
  • Disable ports such as LPT or COM ports.
  • Caused keyboard keys to be remapped.
  • Alter the system time and date.
  • Cause system to hang or freeze randomly.
  • Cause activity on HDD or FDD randomly.
  • Increase file size.
  • Increase or decrease memory size.
  • Randomly change file or memory size.
  • Extended boot times.
  • Increase disk access times.
  • Cause a computer to make strange noises, make music, clicking noises, or beeps.
  • Display pictures randomly.
  • Unusual or undocumented error messages.

Examples of computer viruses

Types of viruses

Below is a list of the different types of computer viruses. Visiting any of the following links gives you further information about that type of virus and its capabilities.

Appending virus
Boot sector virus
Cavity virus
CMOS virus
Companion virus
Encrypted virus
Executable virus
MBR virus
Macro virus
Multipart virus
Non-resident virus
Overwrite virus
Polymorphic virus
Rabbit virus
Resident virus
Spacefiller virus
Stealth virus

Specific viruses

Below is a listing of specific computer viruses and details about that virus. This list is a very small subset of the millions of different viruses that were released since the 1980s.

Alive.3400 virus trojan
Badtrans trojan virus
Blaster worm
Cap virus
CIH virus
E-Morph virus
Excel/Laroux virus
Flor virus
KAK virus
Live Stages virus
Lonely hearts trojan virus
Love letter virus
Melissa virus
Mutation virus engine
Nakedwife virus
Nimda virus
NYB virus
OVER 1644 virusResume worm
Satan Bug Natas virus
Sircam virus
Stoned Empire Monkey virus
Sulfnbk virus
VBS/Gnutella worm
VBS/[email protected] script virus

Can a virus physically damage my computer?

As they are only code, viruses and other malware are only capable of affecting the data on your computer and cannot physically damage it. However, there are viruses designed to physically damage computers or hardware equipment connected to computers. One of the most notable viruses capable of damaging hardware is Stuxnet, which was designed to target industrial equipment, like nuclear reactors.

Can a virus infect a CD, DVD, or another disc?

No, a computer virus cannot infect a CD or DVD, as CDs or DVDs are locked after being created, preventing additional files from being put on that disc. However, if during the creation of the disc, a virus is also copied to it, the disc would become infected.

When was the term "virus" first coined?

The concept of a computer program capable of reproducing itself was first mentioned by John von Neumann in his 1949 "Theory of Self-Reproducing Automata" essay. Later, Fred Cohen in 1983 coined the term virus, describing it in a research paper as "a computer program that can affect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself."

What was the first computer virus ever created?

The first computer virus, known as the Elk Cloner, was written by 15-year old high school student Rich Skrenta in 1982. The Elk Cloner virus spread to other computers by monitoring the floppy drive and copying itself to any floppy diskette inserted into the computer. Once a floppy was infected, it would infect all other computers that used the diskette. A computer that was infected would display a short poem on every 50th boot.

How many computer viruses exist today?

An exact number of computer viruses in existence today is complicated, if not impossible, to determine. In 2018, Kaspersky reported an estimated 187 million different viruses or related threats, including viruses and threats for computers and mobile devices.

New viruses, either original or variants of existing viruses, are created nearly every day. As such, antivirus companies, including Kaspersky, Symantec, and Trend Micro, update their list of known viruses daily.

What is the difference between malware and a virus?

Malware is a description of any malicious software, which includes computer viruses. Malware may also include spyware, Trojans, worms, or any other software designed with ill-natured intentions. See our malware page for further information.

Computer virus myths

Below are some comments we've come across that are common misconceptions when it comes to computer viruses and other malware.

"If I download a file onto a disk, I don't have to worry about viruses."

A file on a diskette or another drive (e.g., USB drive) can still be infected. Many viruses are memory resident and capable of loading themselves into memory once a diskette is placed in the computer it could become immediately infected. Anywhere you can save or edit a file, a virus can infect.

"If I buy sealed software, I don't have to worry about viruses."
"If I buy registered software, I don't have to worry about viruses."

A program disc surrounded by plastic doesn't protect it from a virus. When software is saved onto a diskette or disc, if that computer is infected, the virus could also be saved. Although software from a developer is rarely infected, it's still possible and has happened.

"If I don't download anything from the Internet, I won't get a virus."

Although many of today's viruses and other malware come from downloading files, it's still possible to get infected without downloading. Also, when you are on the Internet, almost everything you view is downloaded to your computer.

"If I only read my e-mail, I will not have to worry about viruses."

Not true; some viruses are distributed through e-mail. Also, files can be attached to e-mail and, if executed, can infect the computer. Today, this is one of the most common ways computer viruses spread around the world.

"If I don't get on the Internet, I don't have to worry about viruses."

Although most viruses spread over the Internet today, a computer virus can infect from a diskette, disc, or USB drive. Also, if a computer connects to other computers over a local network, any other computer on the network with access could infect a computer.

"You can contract viruses from looking at web pages."

True. Although it's more common to get viruses and malware from files you download, it's also possible to be infected by malware upon visiting a website. If a website has a bad ActiveX or JavaScript file, it's possible to spy on your browsing habits and infect the computer with malware and spyware.

What is the full form of "virus," or what is it short for?

The term "virus" is not an acronym. When referring to an antivirus (virus protection), the "anti" is a prefix that indicates a program helps protect against computer viruses.

Antivirus, Hard drive, Hoax, Intruder, Logic bomb, Security terms, Software terms, Trojan horse, Vaccine, Worm, Zoo