Worm vs. Virus: What's the Difference and Does It Matter?

Malware vs. virus vs. worm

When distinguishing a computer virus vs. worm, it’s important to understand the larger overall category of malicious software, or malware. Malware refers to all code and applications written to harm devices or the people who use them. Malware is a broad category that includes spyware, adware, ransomware, and several other types. Viruses and worms are two distinct types of malware, but both can spread rapidly.

Inheriting its name from its biological counterpart, a computer virus spreads quickly from device to device. A virus must be attached to a host, such as an executable file or a document. Viruses infect your system, inserting their code into a program or operating system code and borrowing your computer’s resources to copy and spread.

Viruses and worms both cause damage and copy themselves rapidly. The main difference is how they self-replicate, with viruses requiring the help of a host and worms acting independently.

A virus lays dormant in your system until you execute it or take some other required action, such as enabling macros on an infected DOC file. Once activated, the virus inserts its code into other programs on your device to help copy itself, corrupt your files, damage device performance, and spread to additional devices.

While they do share some characteristics with viruses, computer worms are self-sufficient programs that don’t require a host program or file. Thus, worms do not rely on you to trigger them. Many worms can self-replicate and self-propagate without any human activation. This allows worms to spread extremely quickly, often over a local network and sometimes at an exponential rate.

In brief: Viruses and worms both cause damage and copy themselves rapidly. The main difference is how they self-replicate, with viruses requiring the help of a host and worms acting independently.

Unlike viruses, worms can replicate and spread without any human activation.

While a worm might use a file or program to sneak onto your computer, once it’s in your system, it can spread without the host file and without a human interacting with it at all. A computer worm can also enter your system by exploiting a security vulnerability, which means that you won’t notice it at all. In contrast, a virus spreads from its infected host once it’s been inadvertently activated by the user.

Which is more dangerous?

The impact of both viruses and worms can range in severity from mild to catastrophic.

• Annoyance: Some viruses and worms present merely annoying messages or pop-ups.

• Device performance issues: You may notice slow performance, crashing or freezing, changed settings, and things generally not working as they should.

• Data or money loss; identity theft: Viruses and worms may be able to steal your personal information, leading to identity theft and financial loss.

• Corporate money loss; brand damage: Businesses can also be hit with viruses and worms, leading to stolen data, repair costs, and damaged reputations.

• Large-scale, nation-state attacks: Cybercrime is practiced regularly by state-sponsored hacking groups. Viruses, worms, and other forms of malware can damage government institutions and even nuclear weapons sites.

Usually, a worm is more dangerous than a virus, because it can spread more quickly. For example, a worm could infect all of your email contacts. It could then spread to all of your contacts’ contacts, and then your contacts’ contacts’ contacts, and so on, creating exponential growth with extraordinary potential for damage.

Worms can also spread without notifying you or interacting with you at all, such as through a security vulnerability. The worm can detect any device that contains the vulnerability and then insert itself by exploiting that vulnerability.

How do viruses and worms spread?

As mentioned above, viruses require human interaction, such as you executing an infected program, to spread. Worms can spread automatically, without you doing anything. But how do devices first contract a virus or worm infection? Here are the most common ways:

• Email: The most popular delivery method for malware, email can include infected attachments and malicious links containing viruses or worms.

• Security vulnerabilities: Developers sometimes accidentally leave vulnerabilities in their code, which hackers exploit to insert malware into your system. A worm can scan a network, find all devices with the vulnerability, and exploit them to gain access. 

• P2P file-sharing: If you download music, movies, or apps from unauthorized sources, you might get more than you bargained for in the download.

• Malvertising: Websites, even legitimate ones, can contain infected ads in the form of banners or pop-ups. If you click on them, you can unwittingly download a virus or worm. Even if you don’t click, some of these ads can execute a drive-by-download that infects you anyway.

Blended threats

Theoretically, there are clear distinctions between malware vs. viruses (though the later are a subset of the former), viruses vs. worms, spyware vs. adware, and so on. But in the real world, the lines can get blurry when cybercriminals combine aspects of several different threats.

An example of a blended threat would be using a Trojan horse (malicious code disguised as something beneficial) to sneak in a virus.

WannaCry is a real-world example of a blended threat that caused extreme damage around the world. It combined aspects of a worm and ransomware, infecting victims’ computers via EternalBlue, an NSA-developed exploit revealed in 2017 by the Shadow Brokers hacking group.

Once WannaCry infected a victim’s computer through the EternalBlue exploit, it encrypted their files and demanded a ransom to release them. Then, WannaCry scanned the network to find other devices with the same security vulnerability. As soon as it found one, it installed itself on the new device and repeated the process all over again.

WannaCry spread like wildfire, infecting 10,000 PCs every hour.

WannaCry spread like wildfire, infecting 10,000 PCs every hour. Before it was finally halted, WannaCry brought down several high-profile targets, including hospitals, governments, and universities.

Blended threats bring together the worst parts of different cyberthreats, making them some of the most dangerous examples of cybercrime. WannaCry was no exception — experts estimate that it caused up to $4 billion in damage. And that doesn’t include the fear and stress caused when hospitals and governments saw their computer systems locking up right before their eyes.

How to tell if your device is infected

If you think your device might be infected, look for some of these tell-tale signs of malware:

• Device problems: Slow performance, changed settings, pop-ups, crashes, and freezes can all alert you that something is wrong.

• Increased power consumption or data charges: Some types of malware drain not only your device’s processor, but also your battery and data allowance.

• Demands for ransom: If you contract ransomware through a blended threat, it will be obvious, because a screen will pop up telling you that your files are locked until you pay up.

If you notice any of the above issues, it may be difficult to determine whether you have a virus vs. a worm vs. some other type of malware. Luckily, in most cases the cure is the same: you need a strong virus removal tool.

A robust antivirus program like Avast One will scan your device from top to bottom, discover any rogue code, and eradicate it on the spot. No matter if it’s a virus or worm, Avast One will clean up your device in no time. And with six layers of protection and our AI-enhanced cloud-based system, Avast will prevent malware from infecting you in the future.