What Is The Difference Between A Worm, A Trojan & A Virus?

Some people call any type of malicious software a “computer virus,” but that isn’t accurate. Viruses, worms, and trojans are different types of malicious software with different behaviors. In particular, they spread themselves in very different ways.
Malicious software in general is referred to as “malware.” If you want a catch-all term for bad computer software, malware is the word to use. The good news is that your antivirus program doesn’t care what the malicious software is called — antivirus programs also remove worms, trojans, and other types of malware.

Viruses Infect Other Files

A computer virus infects other files, similar to how a biological virus infects living cells. When you execute a virus by running an already-infected file, the virus infects other files on your system. In most cases, it adds itself to existing .exe files on your system, so it will run when they do.
Some types of viruses can also infect other types of files, such as macros in Word or Excel documents. The virus spreads when an infected file is transmitted to another system. This may happen via an email attachment, removable storage device or a network file share.

Viruses also wreak havoc on your system. In some cases, they may replace existing program files with themselves entirely instead of just adding themselves to the existing programs. They may delete files and announce their presence. Viruses can also take up system memory and cause crashes.
Viruses are dangerous because of how they spread. When a legitimate file moves between systems, the virus comes with it.

Worms Copy Themselves

A worm is a standalone program that doesn’t require user intervention to spread. Worms don’t infect existing files – they spread copies of themselves instead.
Some worms, like the infamous Mydoom worm, email copies of themselves to every address in a computer’s address book. Some of the most dangerous and fast spreading worms, such as the Blaster and Sasser worms, exploit vulnerabilities in network services. Instead of emailing files, they travel over the network and infect unpatched systems that aren’t running firewalls.

Worms that spread over the network can generate a large amount of traffic, slowing down the network. Once a worm is on your system, it can perform the same malicious actions that a virus can.

Trojans Lie In Wait

Trojans are named after the mythological trojan horse. To conquer the city of Troy, the Greeks constructed a wooden horse and presented it to the Trojans as a gift. The Trojans accepted the gift into their city. Later that night, Greek soldiers emerged from the inside of the hollow wooden horse, opened the city gate – and you can imagine what followed.
A trojan horse is the same sort of thing. Trojan horses masquerade as useful software, such as a legitimate program or an illicit crack for a legitimate program. Instead of being well-behaved software, a trojan opens a backdoor on your system.

The trojan’s author can use the backdoor to make your system part of a botnet, use your Internet connection to perform illicit activities that will be traced back to you, download other malware programs onto your system, or do anything else they want.
Trojans don’t try to spread themselves in any way. Trojans must be manually executed by a user.

Other Threats

These aren’t the only types of malware. Here are a few other common ones:

• Spyware – Spyware spies on you. Spyware encompasses everything from “key loggers” that log your keystrokes to steal your credit card information and online banking passwords, to advertising programs that monitor your web browsing activity and send it over the Internet. Spyware is generally designed to make money for its creators. Some antivirus programs don’t detect and remove this sort of software, but Windows Defender, built into Windows 7, does.

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• Scareware – Scareware, also known as crimeware, often appears as a fake antivirus alert on a web page. If you believe the  alert and download the fake antivirus program, it will inform you that you have viruses on your system. The antivirus program asks for a credit card number, insisting on a payment before “fixing” your system. Scareware holds your system hostage until you pay or remove it.

Keep your operating system and other software up-to-date and run an antivirus program to protect yourself from these dangers.
Is there anything else you want to know about viruses, worms, trojans, or other type of malware? Leave a comment and we’ll answer any questions you have.
Image Credit: Infected Computer via Shutterstock, Bacteriophage via Shutterstock, Worm in Email via Shutterstock, Trojan Horse Photo from Alaskan Dude, Hand Reaching Out From Laptop via Shutterstock