Types of Viruses in Cybersecurity | Full Guide to Threats, Examples & Prevention (2025)

Computer viruses have evolved from basic disruptive scripts to sophisticated, multi-stage attacks capable of paralyzing entire networks. Understanding the various types of viruses is essential in today’s cyber-driven environment. Each type behaves differently, infects different parts of your system, and requires specific prevention and removal techniques.

Let’s explore the major types of viruses, how they work, and how you can defend your devices and data against them in 2025.

File Infector Viruses

File infector viruses are one of the oldest and most common forms of malware. These viruses attach themselves to executable files such as .exe, .com, or .dll. Once the infected file is opened, the virus is activated and spreads to other system files.

Example: The famous CIH virus (also known as Chernobyl) was a destructive file infector that could overwrite system BIOS.

Defense Tip: Keep antivirus software updated and avoid downloading executables from unknown sources.

Macro Viruses

Macro viruses target applications like Microsoft Word or Excel. They use scripting languages like VBA (Visual Basic for Applications) to run malicious commands embedded in documents. These viruses often spread through infected email attachments.

Example: Melissa virus infected Word documents and emailed itself to users in the victim’s address book.

Defense Tip: Disable macros by default and be cautious with email attachments.

Boot Sector Viruses

These viruses infect the master boot record (MBR) of a hard disk. When the computer is booted, the virus loads into memory and can take control before the operating system starts.

Example: The Michelangelo virus was programmed to activate on March 6 and overwrite critical parts of the disk.

Defense Tip: Use modern UEFI systems and boot-time protection tools to block these threats.

Polymorphic Viruses

Polymorphic viruses are designed to change their code each time they infect a system. This makes them difficult for traditional signature-based antivirus programs to detect.

Example: Storm Worm was a polymorphic Trojan that evolved its code to stay hidden.

Defense Tip: Use behavior-based detection systems and next-gen antivirus (NGAV) that identify suspicious activity, not just code.

Metamorphic Viruses

Metamorphic viruses go a step further by completely rewriting their code with each iteration, using mutation engines. They do not rely on encryption but instead morph their structure entirely.

Example: ZMist was a known metamorphic virus capable of inserting itself into Windows files.

Defense Tip: Use AI-powered endpoint protection platforms that detect changes in behavior and logic.

Resident Viruses

Resident viruses embed themselves in the computer's memory. Once active, they can intercept system functions and infect files even when the original carrier file is deleted.

Example: Randex and CMJ are known resident viruses.

Defense Tip: Use memory-scanning antivirus tools that detect active threats running in the background.

Non-Resident Viruses

These viruses don’t stay in memory. Instead, they search for files to infect, act, and then disappear. They depend on being re-executed to cause further harm.

Example: Vienna virus was a classic non-resident virus.

Defense Tip: Scan newly downloaded files before running them to stop the virus in its tracks.

Multipartite Viruses

Multipartite viruses attack multiple parts of the system simultaneously, including files, boot sectors, and memory. They are more dangerous because they can spread through more than one vector.

Example: Tequila virus affected both program files and the boot sector.

Defense Tip: Regular full-system scans and boot-level security tools are necessary.

Overwrite Viruses

These viruses replace the content of infected files with their own code, making the original data unrecoverable.

Example: The Trj.Reboot virus is known to overwrite files, destroying them.

Defense Tip: Always maintain secure, offline backups of important files.

Web Scripting Viruses

Web scripting viruses target vulnerabilities in web browsers or plug-ins. They’re delivered via malicious websites or ads and often use JavaScript or Flash exploits.

Example: JS.Fortnight virus used JavaScript on websites to infect systems.

Defense Tip: Use script blockers and disable JavaScript on unknown or untrusted websites.

Network Viruses

These viruses exploit network vulnerabilities to spread across systems. They don’t need user interaction, making them extremely fast in propagation.

Example: Nimda spread through email, open network shares, and web servers.

Defense Tip: Patch systems regularly, use strong network segmentation, and deploy intrusion detection systems (IDS).

Companion Viruses

Instead of infecting existing files, companion viruses create a duplicate program with the same name but a different extension, tricking the OS into executing the virus instead.

Example: A virus may create a .com file to shadow an .exe program.

Defense Tip: Check file types and paths when running programs, especially from external drives.

Encrypted Viruses

Encrypted viruses hide their malicious code using encryption to avoid detection. They contain a small decryptor which is activated to run the payload.

Example: Cascade was one of the early encrypted viruses that also displayed falling characters.

Defense Tip: Use antivirus tools with heuristic analysis and code emulation.

Logic Bombs

Logic bombs are malicious pieces of code that lie dormant until triggered by a specific condition—such as a date, action, or file deletion.

Example: A disgruntled employee might plant a logic bomb to delete files if they’re terminated.

Defense Tip: Monitor critical file changes and set up alerts for unusual behavior or delayed triggers.

Spacefiller Viruses (Cavity Viruses)

These viruses insert their code into the empty sections of executable files without increasing file size. They are stealthy and harder to detect.

Example: The Lehigh virus infected command.com by filling unused space.

Defense Tip: Use antivirus tools that perform deep file inspection, not just size comparison.

Macro-less Office Viruses

Modern attackers embed malicious payloads in Office documents without relying on macros. They use newer features like DDE (Dynamic Data Exchange).

Example: CVE-2017-11882 targeted Office through memory corruption vulnerabilities without macros.

Defense Tip: Keep Office patched and disable external content execution.

Hybrid Viruses

These combine characteristics of various virus types—such as polymorphic and file infector behavior—making them highly unpredictable.

Example: Several advanced APTs (Advanced Persistent Threats) use hybrid viruses as part of multi-stage attacks.

Defense Tip: Integrate zero-trust security models and SIEM (Security Information and Event Management) systems.

Cloud-Based Viruses

With increasing cloud adoption, cloud-native malware targets shared storage, cloud APIs, and services like Google Drive or OneDrive.

Example: CloudSnooper used cloud traffic to bypass firewalls and infect endpoints.

Defense Tip: Apply MFA, cloud access controls, and audit cloud app permissions.

Conclusion

In 2025, the battle against computer viruses is more complex than ever. From macro viruses hiding in spreadsheets to polymorphic threats bypassing antivirus systems, every user—individual or enterprise—needs a strong security posture.

Understanding these virus types is the first step in building cyber resilience. Equip yourself with updated security tools, adopt best practices, and always be cautious with what you download or click. Remember: cybersecurity isn't just software—it's a mindset.

 FAQs 

What are the main types of computer viruses?

The main types include file infectors, macro viruses, boot sector viruses, polymorphic viruses, metamorphic viruses, resident viruses, and multipartite viruses.

What is a file infector virus?

A file infector attaches itself to executable files like .exe or .dll and spreads when the infected file is opened.

What does a macro virus do?

Macro viruses are embedded in Office documents and use scripting languages like VBA to execute malicious commands.

How do boot sector viruses work?

They infect the master boot record (MBR), allowing the virus to load before the operating system starts.

What makes polymorphic viruses dangerous?

They change their code with every infection, making them hard to detect with signature-based antivirus software.

What’s the difference between polymorphic and metamorphic viruses?

Polymorphic viruses encrypt parts of their code, while metamorphic viruses completely rewrite themselves with each infection.

What is a resident virus?

Resident viruses stay in system memory and can infect files even after the original carrier is deleted.

What is a non-resident virus?

Non-resident viruses infect files when executed but do not stay in memory.

What is a multipartite virus?

These viruses can infect multiple system areas—like the boot sector and files—making them harder to remove.

Can viruses destroy files?

Yes, overwrite viruses can erase or replace file content, often making data unrecoverable.

What is a web scripting virus?

It uses malicious JavaScript or other code in web pages to infect visitors' browsers.

How do network viruses spread?

They exploit vulnerabilities in networks or open shares to infect multiple systems rapidly.

What is a companion virus?

It tricks the system by creating a duplicate file with the same name but a different extension like .com or .bat.

What is an encrypted virus?

Encrypted viruses hide their code with encryption, needing a decryptor to execute.

What are logic bombs?

They activate under specific conditions like a date or the deletion of a file.

What is a spacefiller or cavity virus?

It hides in unused portions of executable files without changing their size.

Are there macro-less Office document viruses?

Yes, some use features like Dynamic Data Exchange (DDE) to launch attacks without macros.

What are hybrid viruses?

They combine multiple virus characteristics (e.g., boot sector + polymorphic) for broader impact.

Are there viruses that target cloud environments?

Yes, cloud-based malware infects shared storage and cloud applications like Google Drive or OneDrive.

How do viruses spread today?

Through emails, USBs, malicious websites, software downloads, and network connections.

What’s the difference between a virus and malware?

Viruses are a type of malware designed to replicate and spread. Malware is a broader category that includes viruses, Trojans, ransomware, etc.

Can antivirus software detect all viruses?

No, especially not advanced polymorphic or metamorphic viruses. Behavior-based detection tools are more effective.

How do I prevent computer viruses?

Use updated antivirus software, avoid unknown downloads, patch systems regularly, and practice safe browsing.

Can a virus infect smartphones?

Yes, mobile malware can infect Android or iOS devices, often through malicious apps or phishing.

What’s an example of a famous computer virus?

The Melissa virus (macro virus) spread rapidly through infected Word documents in the late 1990s.

Are computer viruses still a threat in 2025?

Yes, viruses have evolved and are often used in cyberattacks involving data theft or ransomware.

What tools detect virus infections?

Antivirus software, endpoint detection and response (EDR), and system monitoring tools help identify infections.

Can you recover from a virus attack?

Yes, with the right backups, removal tools, and cleanup strategies, recovery is possible.

What is a stealth virus?

Stealth viruses hide their activity from detection tools by altering file sizes and timestamps.

Are viruses used in ransomware attacks?

Sometimes. Viruses may open doors or assist in delivering ransomware payloads.

What’s the best defense against modern viruses?

Layered security: antivirus, firewalls, behavior monitoring, employee awareness, and regular system updates.