What Is a Computer Virus? Understanding How It Works and Major Types

People often say their computer has a virus whenever it slows down or behaves strangely on the screen. In cybersecurity, however, when we ask “what is a computer virus?”, we are talking about one specific type of malicious program, not a catch-all term.

Read this blog to understand what a computer virus really is, how it behaves on a system, and how it differs from other kinds of malware.

Key Takeaways

• A computer virus is a type of malware that attaches to a host file or program and spreads when that host runs. It needs user or system actions, such as opening an infected file, to move further.
• Most viruses follow the same pattern: they infect a host, activate, copy themselves into more files or system areas, and then deliver a harmful payload that can damage data, steal information, or abuse system resources.
• Viruses usually spread through everyday activities like downloading untrusted software, opening malicious email attachments, using infected USB drives, or triggering software vulnerabilities.
• You should suspect a virus if your computer becomes slow or unstable, shows strange pop ups or unknown apps, changes files or settings on its own, or has unusual network or account activity.

Computer Virus Definition

A computer virus is malicious software that attaches itself to a legitimate file or program. When that host runs, the virus code runs too and can copy itself into other files or areas of the system. In simple terms, a computer virus is a self-replicating program that hijacks normal files or processes.

This is the core definition of a computer virus used in security. A virus always needs some kind of host. The virus code hides inside that host and runs when the host runs. The host might be an application, a document, a script, or even part of the system boot process.

Recent summaries of malware research estimate over 1 billion distinct malware programs in circulation, with around 560,000 new malware samples detected every day across security vendors. This shows how large and active the overall malware landscape is, even though a virus is just one type of threat.

Two traits separate a true virus from many other threats:

• It replicates by inserting its code into other files or system areas.
• It needs some form of user action or normal activity to spread, such as opening an infected file or starting an infected program.

Not every piece of malicious software fits this pattern. Many modern threats spread in other ways and do not attach to files in the same style. People still use the phrase “computer virus” as a catch-all, but security tools treat viruses as one type of malware among many.

How Do Computer Viruses Work?

To understand how computer viruses work, it helps to know that most of them follow a similar life cycle, even if the details differ by family. They usually move through four broad stages: initial infection, activation, replication, and delivery of the harmful action (the payload).

Initial Infection

The first step is getting the virus code onto a system. The code usually arrives inside another file, such as a document with hidden macros, an installer, a script, or a program that appears useful.

When that host file reaches a device, nothing obvious may happen. The virus can remain dormant while the file behaves as the user expects, for example by displaying a document or starting an installer. The malicious part stays in the background.

Activation of the Virus

At some point, the virus code runs. That might happen as soon as the infected file opens, or only when certain conditions are met, such as a particular date, a number of system restarts, or the presence of a network connection.

Simple viruses run their code as quickly as possible. More advanced families wait for a trigger. Delayed activation makes detection harder and keeps the attack less obvious.

Replication

Once active, a virus tries to make copies of itself. This is the defining feature of a PC virus. It searches for new files, folders, or system areas to infect, then modifies those targets to include its own instructions.

A virus might focus on executable program files, macro-enabled documents, common script folders, or system components such as the boot sector or master boot record. Replication gives the virus staying power. Even if one infected file is removed, others can still carry and spread the code.

Payload and Damage

The “payload” is the part of the virus that causes visible harm or abuse. Some payloads are noisy and disruptive; others are quiet and focused on theft or control.

A virus payload may delete or encrypt files, corrupt data, log keystrokes, spy on activity, open a backdoor for remote access, or add the device to a botnet used for spam, attacks, or cryptocurrency mining. Some viruses also weaken security settings so that additional malware can be installed.

Many viruses do not cause damage immediately. They may spend time spreading or collecting information before any clear symptoms appear.

How Do Computer Viruses Spread?

The way viruses spread depends on how they move from one host to another. Several routes show up again and again across different virus families, and understanding how to get a virus by accident helps you avoid those risky situations. 

Infected Files and Software Downloads

Viruses often hide inside files downloaded from the internet. These might be cracked or pirated software, games or tools from untrusted sites, or even legitimate programs that attackers have tampered with.

When the user runs the infected program, the virus activates with it. If that file is later shared through cloud storage, messaging apps, or removable drives, each new device that runs it can become infected.

Email Attachments and Messages

Email is still a major channel for spreading viruses and similar malware. Recent data shows that email is still the main delivery method for malware: about 88% of malicious files are sent by email, and HTML attachments now make up most of those dangerous files.

Attackers send attachments that look like invoices, forms, or other everyday documents. The malicious file may be a macro-enabled document, a compressed archive, or a script or executable with a misleading name.

Once the victim opens the attachment and enables active content, the virus code runs. Some families also use the victim’s address book to send copies of the message to new recipients, allowing rapid spread through organizations.

Removable Drives and Local File Sharing

USB drives, external hard drives, and shared folders give viruses a convenient way to move between computers. An infected system can write compromised files to a removable drive, which then carries those files to another machine.

Shared folders in homes or offices can create similar problems. If a virus infects files stored on a network share, any device that uses that share may be at risk. Older viruses relied heavily on this pattern; newer ones still use it alongside other methods.

Exploits and Vulnerabilities

Some viruses also take advantage of software flaws. They exploit vulnerabilities in operating systems or applications so the virus code can run when a user visits a compromised website, opens a specially crafted file, or connects to a vulnerable service.

The virus still needs to run on a host system to keep spreading. Once it does, it can infect local files, common folders, or system components. The exploit simply provides a more direct way to get in.

Because so many infections start with everyday actions like opening files or downloading apps, computer virus prevention focuses on making these routine behaviors safer and using security software to block threats before they spread.

What Do Viruses Do to Your Computer?

The impact of a virus can range from a mild slowdown to complete loss of data or control. The exact effects depend on the virus family and its payload, but several patterns are seen again and again on infected systems.

Slower and Less Stable Performance

Many viruses make a computer feel slow and unreliable. They consume CPU and memory, start hidden background processes and generate extra network traffic. Over time this can lead to freezes, crashes and longer delays when starting, using or closing programs.

Damaged or Changed Files

Viruses often interfere with files and applications. Documents and projects can become corrupted, important data may no longer open and some programs may fail to launch. System settings may also be altered in ways that make the device unstable, easier to exploit or harder to use safely.

Theft of Data and Accounts

Data theft is a common goal in modern attacks. A virus can record keystrokes, watch network traffic or search for stored passwords and authentication tokens. Criminals can then use this information to sign in to accounts, move money, reset passwords or impersonate the victim on key services.

Use of Your Device in Larger Campaigns

In many cases the infected computer is turned into a tool inside a wider criminal campaign. It may send spam, help launch denial of service attacks or silently mine cryptocurrency for the attacker. The owner often only notices constant high resource usage, noisy fans or unusual spikes in network activity.

Ransom and Extortion

Some viruses play a role in extortion. They encrypt files or lock access to the system and display a ransom note demanding payment for a key or unlock code. These incidents are usually described as ransomware attacks, but many ransomware families still use virus-style techniques to move between systems and reach more victims.

How to Know If Your Computer Has a Virus

If you’re wondering how to know if your computer has a virus, remember that no single symptom proves an infection, but several changes together can be a strong warning sign.

Slow, noisy, or unstable system

Your computer suddenly takes much longer to start, open programs, or shut down. Fans run loudly, and CPU or memory use stays high even when you are not doing much.

Strange pop-ups and apps

You see new pop-ups, fake security alerts, odd error messages, or browser tabs opening on their own. Toolbars, extensions, or apps appear that you do not remember installing.

Files and settings changing

Some documents no longer open, new unknown files or shortcuts appear, and existing shortcuts lead to the wrong programs. Settings change without you making those changes.

Unusual network or account activity

Your internet connection is busy for no clear reason. Other devices on the same network have similar problems. You receive login alerts, password reset emails, or complaints about strange messages sent from your accounts.

These signs do not prove there is a virus on computer systems, but if you notice several of them together, it is sensible to run a full scan with trusted security software.

Types of Computer Viruses

When people talk about types of computer viruses, they often mix technical categories with practical descriptions. Security experts define many sub-types, but a few broad groups are especially useful for understanding how viruses behave.

File Infector Viruses

File infector viruses attach themselves to executable files. They change the program so that the virus code runs when the program starts. As these files are copied, moved, or installed in new locations, the virus travels with them and can spread to other systems via shared drives or downloads.

Macro Viruses

Macro viruses target documents that support scripting, such as word processing or spreadsheet files. The malicious code hides inside a document or template. When the file opens with macros enabled, the virus runs and may infect other documents or templates on the same system.

Boot Sector Viruses

Boot sector viruses infect the part of a drive that contains start-up code. They were especially common when systems booted from floppy disks, but similar techniques can still be used. Because they run very early in the boot process, they can be difficult to detect and remove.

Multipartite Viruses

Multipartite viruses use more than one infection method at the same time. A single virus may, for example, infect both executable files and the boot sector. This makes cleanup more complex, because removing one part of the infection does not necessarily remove the others.

Polymorphic and Metamorphic Viruses

Polymorphic viruses change parts of their code, or the way they are encrypted, each time they replicate. Metamorphic viruses go further and can rewrite larger sections of their own structure. The behavior remains similar, but the code looks different from copy to copy, which makes simple signature-based detection harder.

Resident and Non-Resident Viruses

Resident viruses remain in memory after their host program runs. They can then infect other files during normal system use. Non-resident viruses act only while the infected program is running and stop when that program closes, until it is started again.

Summary Table of Common Computer Virus Types

These groups cover many of the most common types of viruses. Individual families may combine features from several rows.

Computer Viruses vs Other Malware, Worms, and Trojans

Viruses, worms, Trojans and other types of malware are related terms, but they do not mean the same thing.

1. Malware: Malware is the broad category. It covers any software that is designed to harm, exploit or misuse a system. Viruses, worms, Trojans, ransomware, spyware and many other threats are all types of malware.
2. Worm: A worm is a self-contained program that spreads on its own, often across networks. It does not need to attach to a file. It moves by exploiting vulnerabilities or weak passwords and copies itself directly to new systems.
3. Trojan: A Trojan looks like a useful or harmless program but hides malicious behavior. It does not spread by infecting other files. Instead, it depends on tricking users or systems into running it. Once active, it can install more malware, steal data or open a backdoor.

The table below highlights some of these differences:

FAQ About Computer Viruses