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What are computer viruses? Definition and types of viruses

Few things are worse than sitting down to work and realizing your computer has been infected with a virus. How did this happen? And how can you fix it when your computer has become virtually unusable? If this situation sounds familiar, don’t worry. You are not alone. But before you try to fix the problem, it’s best to get to know the enemy. Here’s everything you need to know about virus types, causes, and removal.

What are computer viruses? Definition and types of viruses

What is a computer virus?

A computer virus is a type of malware, an illegal and malicious application that infiltrates a computer and often affects its behavior and performance. Some viruses are designed to do damage on the target device such as destroy data, while others are built to spread throughout the network and perform more delicate tasks such as gathering sensitive information.

Is computer virus malware? It’s important to note that viruses and malware can be used interchangeably, but the words generally mean different things. Malware is the broad term used to describe all malicious programs including but not limited to viruses, ransomware, rootkits, spyware, and adware.

How does a computer virus work?

Computer viruses work in a variety of ways as some are triggered immediately, while others may lay dormant until they’re ordered to strike. If a virus is designed to execute as soon as possible, it can have an immediate effect on the computer’s behavior, potentially revealing itself to the user.

When cybercriminals are trying to stay under the radar, the virus is executed in several stages. For example, after the dormant phase, the virus may try to replicate itself and spread silently to other devices on the network. Eventually, the virus is triggered automatically or waits until the hacker executes it remotely and releases the malicious code (also known as payload).

How do computer viruses spread?

The way computer viruses spread has changed over the last three decades completely. Initially, computer viruses had to be transferred manually using floppy disks. As technology advanced, the use of CDs and USB thumb drives became a major cause of viruses. Now, most viruses spread through the internet through such methods as fake software updates, illegal games, and email attachments. However, viruses can still be spread physically.

Types of computer viruses

Here are the most common types of viruses:


Sometimes, classified as its own type of malware, trojans are computer viruses that, as the name suggests, are disguised as a legitimate app. After it gets access to the system, the trojan executes.

Computer worm

A computer worm can self-replicate and spread across computer networks. Since it doesn’t need a host program to multiply, a worm can do significant damage to a computer by deleting files, overloading networks, or stealing information.

File infector

A file infector virus is another common virus that infects .exe or .com files. When it’s executed, the virus can damage the data, replicate, and spread to other files. File infector viruses use various stealth techniques and may be difficult to detect.

Resident virus

A resident virus loads itself into the computer’s memory and continues to infect it even if the program it came with is deleted. Detecting and removing a resident virus can be difficult because it can replicate inside the computer’s memory.

Boot sector virus

As the name suggests, this virus infects the computer’s master boot record (MBR) in order to load before the antivirus has a chance to act. After the computer boots up, boot sector viruses load themselves into the memory and continue to infect other parts of the storage device.

Multipartite virus

A multipartite virus is able to infect a computer’s memory as well as executable files. It uses a variety of ways to spread, such as infecting memory devices by changing the code of applications. Because of its ability to infect multiple areas of the system, this virus may be extremely difficult to detect and remove.

Direct action virus

While direct action viruses are known for infecting the device and then deleting themselves, they are not always programmed to do that. Upon execution, this type of virus often scans the system for specific files, such as .exe, .bat, and .com, and infects them. Direct action viruses are only active as long as the infected program is open.

Computer virus examples

Nobody wants to deal with rootkits, trojans, or ransomware. That’s why they’re often bundled together into the same category and referred to as “viruses” even though only some are. Let’s look at a few examples and make a distinction of what is and is not a virus.

Is a trojan a virus?

Yes, a trojan can be a virus. A trojan is any malicious program that is trying to sneak into the system by pretending to be a legitimate program. So if a computer virus is disguised as a real software file, then yes, it is a trojan virus.

Is ransomware a virus?

No, ransomware is not a virus. Ransomware is a type of malware that infects the computer or system, encrypts its files, and demands a ransom to give access back to the owner. Even though it’s not a virus, viruses can be utilized in a ransomware attack.

Is a rootkit a virus?

No, rootkit is not a virus. A rootkit isn’t designed to spread or infect files. Instead, it gives hackers access to the system.

Is a software bug a virus?

No, a software bug is not a virus. It’s an error in software code, causing the program to behave unexpectedly. However, bugs can be exploited in cyberattacks unless they’re patched by the developers through software updates.

Common signs your computer is infected with a virus

It’s natural for devices to start feeling more sluggish as they age. But when a virus infects a computer, usually the performance drops are sudden and significant. In addition to performance issues, your computer may also start acting up, like showing random pop-ups and changing the browser homepage.

Here are the most common signs that a virus has infected your device:

  • Sudden performance issues. Computer viruses need resources, so if they start self-replicating and infecting files, you should notice. Your device will run slow, and programs and web pages may take longer to load.
  • Pop-ups. Pop-ups and adware, or malvertising, are very common for less delicate types of viruses.
  • Corrupted files. While files can get corrupted due to power outages and hardware problems, corrupted files can also be a sign that a virus is spreading inside your computer.
  • Crashing OS. If viruses start infecting system files and computer memory, it’s likely that your operating system will crash frequently.
  • Misbehaving programs. Infected by a virus, applications may start or close by themselves, or they may stop working altogether.

How can I prevent computer viruses?

While preventing viruses requires both luck and common sense, the latter should keep you out of harm’s way. This means understanding that the internet is not always a safe place, so every click, email, or attachment may lead to a virus. The good news is that they usually don’t, and, when they do, you can recognize them before they have a chance to harm your computer.

Here are common ways to prevent computer viruses:

  • Be careful with email. Email is often utilized in cybercrime, especially phishing. But often, you can recognize it by the inclusion of unwanted attachments, spelling mistakes, or dodgy email addresses. But remember that some viruses spread through email programs, so just because you received an email from somebody you know, don’t assume it’s automatically safe.
  • Update your software. Hackers often exploit software vulnerabilities that developers race to fix. To limit the risk of software used against you, keep it up to date.
  • Use a VPN. A VPN makes browsing the web on public networks more secure Additional safety features such as NordVPN’s malware protection tool Threat Protection Pro may also help protect your device from malware.
  • Double-check if you’re downloading software from official and reputable sources.
  • Download a secure browser. The market has plenty to offer in terms of convenient web browsing. But that doesn’t mean you should sacrifice privacy. Find a web browser that puts your privacy and security first.
  • Trust your antivirus software. Keep it on and updated at all times. It may not seem to be doing much in the background, but it’s constantly monitoring computer processes and your downloads to ensure your computer doesn’t get infected.

How can I remove a computer virus?

The best way to remove a computer virus is to prevent it from infecting your computer in the first place. The second best way is to scan your computer and hope your antivirus handles the task.

If your antivirus recognizes the virus, but performance issues persist, it’s worth researching the name of the virus online. Specialized tools to help users remove that virus may already have been developed, or you may be able to find the steps for how to deal with it.