Difference between Virus, Worm and Trojan Horse

Viruses, Worms, and Trojan Horses are types of malicious software that infiltrate a system and try to create havoc, however what sets them apart is the way they attack, spread, and replicate. Read though this article to get an overview of how a virus is different from a worm, and what exactly is a Trojan Horse.

What is a Virus?

A virus is malicious software (malware) made up of little bits of code attached to legitimate programs. When that software is launched, the virus is launched as well.

Viruses are malicious programs that infect computer files and spread without the user's knowledge. The most common virus infections are spread via e-mail attachments that activate when opened. As infected e-mails are forwarded to multiple people, the virus's vicious cycle continues. Viruses can also be propagated through shared media, such as USB flash drives.

Viruses are responsible for widespread and major computer systems and file loss. They were initially intended as pranks. Anti-virus software can assist prevent, block, or delete viruses that have already been installed.

What is a Worm?

A worm is a harmful software (virus) that replicates itself as it moves from computer to computer, leaving copies of itself in each computer's memory.

A worm finds a computer's vulnerability and spreads like an illness throughout its associated network, constantly looking for new holes. Worms, like viruses, are spread by e-mail attachments from seemingly trustworthy senders. Worms then spread through an e-mail account and address book to a user's contacts.

Some worms reproduce and then go dormant, while others inflict harm. The Worm's code is referred to as payload in such circumstances.

What is a Trojan Horse?

A Trojan horse is malware that disguises itself as a genuine program and downloads it onto a computer. A Trojan horse gets its name from how it's delivered: an attacker often uses social engineering to disguise malicious code within genuine software.

One of the critical characteristics of a Trojan is that it cannot replicate itself, and a user has to install it themselves. It produces a chance for another PC to fully control the infected PC and replicate to harm the host computer systems or steal data. A Trojan horse will damage your computer once it is installed or used, but it will look to be helpful software at first glance.

A Trojan virus spreads by spamming genuine-looking e-mails and attachments to the inboxes of a large number of users. Trojans can also infect devices when cybercriminals persuade people to download malicious software. The malicious software could be disguised in banner advertisements, pop-up ads, or website links.

Beast, Zeus, The Blackhole Exploit Kit, and Back Orifice are example of some famous Trojan horses.

Difference between Virus, Worm and Trojan Horse

The following table highlights the major differences between Virus, Worm, and Trojan Horse −

Virus Worm Trojan Horse
A Virus is a computer program or software loaded, either deliberately or unknowingly by the user. It connects to another software/program to execute unanticipated tasks when the system's actual program is running. A Worm is a computer program similar to a virus that does not communicate with other system programs but multiplies and runs itself to slow down and damage the performance of the system. Trojan Horse is a hidden piece of malware that steals sensitive information/data from a user's system and sends it to another location across the network.
Viruses cannot be operated remotely since they are installed on the target machine or by the user inadvertently. Worms can be controlled by the remote because they can open a back door to the host. Trojan Horse can also be operated remotely, much like worms via the network.
Viruses, like worms, cannot replicate themselves. Viruses also propagate at a moderate rate. Worms replicate themselves in the system and propagate quicker than viruses and Trojan horses. In comparison to viruses and worms, a Trojan Horse spreads slowly.
The primary goal of a virus is to alter or erase system data. Worms aim to degrade system performance and slow it down by eating system resources. The Trojan horse virus, much like in the story, disguises itself as normal software and steals crucial information
Viruses use executable files to spread. Worms take use of system flaws to carry out their attacks. Trojan horse is a type of malware that runs through a program and is interpreted as utility software.


Both Viruses and Worms spread from system to system and corrupt user files, documents, and other data. Although a virus can replicate inside a system after infecting it, but it cannot spread to other systems without human intervention. Worms, on the other hand, do not attach themselves to the host; instead, they exploit the network vulnerabilities to spread.

A Trojan Horse is different from a Virus or a Worm in the way it is delivered. Note that a Trojan Horse cannot replicate itself. They are normally installed by users with the sole intention of stealing critical data.

It is difficult to deal with Viruses, Worms, Trojan Horses, once they affect the system, hence it is always advisable to have a robust antivirus solution and firewall installed on your device. As a security precaution, you should always download computer programs and Apps from official trustworthy sources.