Computer Virus Definition... Viruses & How They Affect Us

What are viruses? Well, usually, 'computer virus definition' is used as the collective name for various types of viruses, worms and Trojan horses.

Computer viruses are just like death and taxes... if we own a PC, we WILL be infected... unless we have the best antivirus software installed.

A common misconception is that worms and Trojan horse applications are viruses, but they aren't. We'll cover what each are and their differences, as follows...

1. Computer Virus Definition... What Are Viruses and Worms?

A virus is a man-made computer program that infects a file or program on our computers. Each time the infected program is run, the virus is also triggered. It replicates or spreads itself by infecting other programs on the same computer.

The explanation from is typical of a computer virus definition...

"A computer program that replicates on computer systems by incorporating itself into shared programs. Viruses range from harmless pranks that merely display an annoying message to programs that can destroy files or disable a computer altogether. Whether they're considered malicious or malevolent, all viruses spread rapidly. For example, from one computer to millions of others around the world, infecting machines and causing them to crash."

A worm is also a man-made program that replicates itself. However, unlike a virus, it does not infect other program files on the computer. Instead, a worm can spread itself automatically to other computers. Cleverly, it does this by sending a copy of itself through email, over a network and via Internet Relay Chat (IRC) to other computers.

There are thousands of different kinds of viruses but they form distinct groups. They all operate differently and affect our computers and the information contained on them in different ways.

The table you see below gives you a feel for some of the types of viruses...

The Types of Viruses


Virus Type

What It Does

How Affects Our PC


  Program or File Virus

Infects executables (other programs, with affixes such as EXE, BIN, COM, SYS)

Destroys or alters programs and data.

Boot sector Virus

Infects boot sectors on hard and floppy disks

Destroys or alters programs and data.

Multipartite Virus

A hybrid of a program and boot sector virus

Destroys or alters programs and data.

Macro Virus Triggers on a command in  Microsoft Office Commonly affects Word & Excel

  Stealth Virus

Uses various tactics to avoid detection.

Destroys or alters programs and data.


Polymorphic Virus


Uses encryption to foil detection, so that it appears differently in each infection.

Destroys or alters programs and data.


Computer Virus Definition... How We Get Infected and The Effects

Viruses and worms can infect network servers, desktop computers, laptops, tablets and mobile 'phones... they pose one of the most serious Internet security risks, reaching our PCs from email, web sites, downloaded files and loadable media, such as USBs, CD-ROMs and DVDs.

Unfortunately, the effects of an infection are pretty unpleasant. The virus or worm can,

  • Disable the computer

  • Add, modify or delete files or reformat the hard disk. A worm eats up storage space and slows down the computer, but won't alter or delete files.

  • Steal addresses held in our computer to send on virus-infected emails to our friends, colleagues, clients or customers

  • Send spam (unsolicited, bulk email) to those in our mail address books and other users

Computer Virus Definition... How Do We Protect Ourselves?

Viruses and worms are detected and removed by antivirus software. But beware... we can still get infected if we haven't got a top performing AV program or haven't updated our virus definitions.

For more information on antivirus software, see Computer Virus Definition: The Best Antivirus Programs

You'll see from this article that you need one of these AV software products...

OR, if you prefer an Internet Security Suite, then one of these two, of equal performance, will fit perfectly...

2. Computer Virus Definition... What Is A Trojan Horse?

Although Trojan horse programs are categorized as viruses, they are not true viruses, since they do not replicate. It is a malicious program disguised as something benign, such as a screen saver or built into a free download.

Unfortunately, Trojan Horses are often hooked on to free downloads bacause many of us cannot resist a free offer! However, this does free downloads a dis-service, as most are genuine and malware-free

When loaded onto our machine, a Trojan horse can capture information from our system -- such as user names and passwords -- or could allow a malicious hacker to remotely control our computer. They really can do significant damage to our privacy and security, so you need software that deals effectively with this widespread and difficult-to-detect malware.

Computer Virus Definition... How Do We Get Infected?

Trojans are one of the sneakiest of the online risks. They are often downloaded on the back of a free program (freeware) that has some value to the user -- a free game, software program or music, for instance.

How Does A Trojan Affect Us?

A Trojan installed on a computer allows that PC to be entered by any remote user that has the access code to the Trojan.

The remote attacker can enter the computer undetected when online, to access or destroy any information stored. Alternatively, the Trojan can be programmed to automatically send any information on our PCs back to the attacker. This could include,

  • Client or customer information or other business data

  • Credit card details

  • Passwords for access to your online bank, ISP or web services

  • Information we would rather remain hidden

  • Email addresses, which may be used for spamming

  • Children's names, photographs, ages or other personal details held on the computer

  • Distributed Denial of Service (DDOS) attack on other computers via the victim's computer

Yes, vicious, invisible programs that you must protect against!!

Computer Virus Definition... Trojan Protection

Like viruses and worms, Trojans are detected and removed by AV software. However, you'll see in Which Is The Best Antivirus Program?, that there are only a few AV programs that are effective at detecting and removing Trojans.

You'll see from this article that you need one of these AV software products...

OR, if you prefer an Internet Security Suite, then one of these two -- of equal performance -- will fit perfectly...

Computer Virus Definition... Supporting Articles


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