Virus, Worms and Trojan Horses

What is the difference between a virus, a worm and a Trojan horse?

Virus, Trojan Horse and Worm are terms often used interchangeably. They are all Malware, after all. But they are not the same.

Trojan Horses, viruses and worms are all malicious software programs that frequently cause major damage to personal computers (and to the servers we all use to connect to the internet and do our work.)

Understanding a bit about the differences between worms, Trojans and viruses can help you protect or even save your computer system from costly damage.

There is more about these terms, and others, on our glossary page.

This page here is to explain in one place the main similarities and differences between these three pests.

The Computer Virus

A virus on your computer will attach itself to a file, folder or program. The virus maker hopes it will spread from your computer to another computer.
Viruses on your computer are not unlike organic viruses: they infect and they spread.
Some computer virus files are mild and cause minimal damage, while others can wipe out every file on your system, and damage your hardware beyond repair.
Virus files usually attach themselves to programs that a user will run, i.e. your executable (.exe) files. This means the virus can exist on your computer, waiting, but not infect your machine until you run or open the program.
Virus files on your computer system cannot usually spread unless human action is involved. That is, you might actually run the infected program, then later unwittingly spread the virus program by sharing files that are infected, e.g. by way an e-mail attachment.
More about Viruses in the glossary.

Computer Worms.

Computer worms are similar to virus files by design, and are considered to be another class of a virus file. Worms also spread from one computer to another, but differ from viruses because worms can travel and spread without human intervention.

Worms spread during the transport of files or information on a computer or network system. Nobody needs to run an infected program: the worms spread without human assistance.
One of the most prevalent dangers from a worm is its ability to self-replicate on your computer system. Instead of your computer sending out one worm, it can send out thousands of copies. This self-replication can cause devastation on your computer system. An example: a worm can send a copy of itself to each person in your address book. The worm then replicates itself and sends itself out to each of those address books – belonging to the people receiving the worm.
Computer worms cause great devastation with, very little human intervention, by spreading, and replicating themselves over and over again.
Because of the worm’s ability to move across networks a further effect is that the worm eats away at the system memory or a network's bandwidth. What eventually happens is that a computer user's system, internet servers and network servers can be slowed down or even brought to a standstill.
Some worms are even designed to tunnel into a computer system and turn computers into zombies, i.e. machines that can be remotely controlled (possibly by identity thieves and child pornographers).
Computer users have had their credit card numbers stolen and their money spent. Other victims of computer worms have had their PayPal accounts drained. Still other unsuspecting computer users have lost the ability to even boot up their computers.
It is horrendous to think about people losing their privacy and money just because their computers had no protection against these known predators.
More about Worms in the glossary.

Trojan Horses

The Trojan Horse can be as devastating on your computer system as its namesake was on its target.
At first the Trojan Horse may appear to be harmless, maybe even useful, software. In fact once installed or running on your system, its damage is done.
Computer users who receive a Trojan Horse innocently open the file as they are usually led to believe they are receiving a legitimate program or file from someone they know or trust.
After a Trojan has been activated on your computer system the damage can vary. There are some Trojans that have been developed to be a nuisance and can do mischief like changing your desktop icons, others are designed to wipe you out by destroying your computer files, folders and programs.
Backdoors to computers are also created by Trojans. This gives the malicious user complete admittance to your computer system. Malicious admittance allows attackers to obtain your personal and confidential data.
Trojans do not replicate themselves like viruses and worms.
More about Trojan Horses in the glossary.

Blended Threat

Also very dangerous is the 'blended threat'.
This is a sophisticated assault that combines some of the means of attack: malicious code, viruses, worms and Trojan horses.
Blended threats take advantage of the weakest links on servers and the internet to launch, transmit and propagate an attack. Blended threats spread quickly and can cause severe destruction and widespread damage.
Blended threats attack by exploiting multiple vulnerabilities. Blended threats are by far the worst assault your system can suffer. An example of the type of damage a blended threat can cause would be the creation of a back door and launching of a DOS (denial of service) attack. A blended threat can damage several components of the network with one keystroke. Blended threats are also designed to require no human to intervene in order to propagate.

Protect your computer by installing updates to your operating system and anti-virus software.
It is easy to prevent a loss you can’t afford.

Quality anti-virus software protects you from worm, virus and trojan horse threats.

Good quality internet security suites will aim to protect you from adware and spyware – and viruses, worms, trojan horses and threats that spread by email.
See on this site for internet security suites from the top anti-virus specialists. E.g. see on our anti-virus pages for:

– Kaspersky security tools.

– BitDefender security tools.

– Panda security tools.