Guide to Computer Viruses: Types and Prevention

What Is a Computer Virus?

A computer virus is a specific type of malicious software (malware) engineered to alter the way a device operates. It operates by inserting its own corrupted code into legitimate programs, system utilities, or macro-supported documents. Once triggered, the virus begins its replication cycle, copying itself across the operating system and creating "infected parts" that can compromise or corrupt critical data.

Unlike other forms of malware, a true virus cannot spread entirely on its own; it requires a host program to be executed or human interaction to jumpstart its infection chain.

13 Different Types of Computer Viruses

1. Spacefiller (Cavity) Virus

While most file infector viruses simply append their payload to the end of a file, spacefiller viruses search for empty blocks of code inside the file itself. By hiding inside these "cavities," the virus can compromise a program without increasing its file size, helping it bypass standard size-based anti-detection protocols.

2. Encrypted Virus

These threats wrap their core malicious code in custom encryption, appending a tiny decryption module to the package. Because the body of the virus looks different in every instance, signature-based antivirus scanners find them incredibly difficult to track. The software usually catches them only when they decrypt themselves in the system memory to execute.

3. Companion Virus

Instead of modifying existing files, a companion virus creates a duplicate file with a slightly different execution priority (such as creating a .com file to accompany a legitimate .exe file). When the user attempts to run the target program, the operating system accidentally prioritizes and runs the companion virus first.

4. Network Virus

Network viruses specialize in spreading across Local Area Networks (LANs) and corporate intranets. They aggressively sweep through shared network resources, such as open drives and public folders. Once they compromise a new terminal, they instantly scan the network topology to find their next target. Famous examples include the Nimda and SQLSlammer strains.

5. Macro Virus

Macro viruses target applications that use automated mini-programs known as macros, such as Microsoft Word or Excel. The virus embeds its script into document templates. When an unsuspecting user opens the file and clicks "Enable Macros," the code triggers. To fight this, modern Office systems block macros by default, forcing hackers to rely on deceptive social engineering tricks to make users enable them.

6. File Infectors

This classic category of viruses hitches a ride on standard executable file formats, primarily targeting extensions like .exe, .com, .sys, or .ovl. The moment a user executes the compromised application, the virus launches into the system background alongside the program.

7. Nonresident Virus

A nonresident virus utilizes a dedicated "finder module." Every time the infected host file runs, the finder module actively searches the local hard drive for clean target files, infects them on the spot, and then terminates its active process rather than lingering in the background.

8. Stealth Virus

Stealth viruses hide from security scanners by intercepting requests directed at the operating system. If an antivirus scanner checks a file for changes, the stealth virus intercepts the command and feeds the scanner a clean, uninfected mock version of the file structure to trick it.

9. Sparse Infectors

To avoid triggering immediate red flags, a sparse infector restricts how often it activates. It might choose to only infect every 20th executed file, target files within strict size limits, or only attack files starting with specific letters. By moving slowly, it delays discovery for as long as possible.

10. Overwrite Virus

An overwrite virus is purely destructive. Rather than hiding inside an application, it systematically overwrites the existing legitimate file data with its own code. The original application code is permanently destroyed, making file recovery impossible without a clean backup.

11. Boot Sector (System Record) Infectors

These threats target the Master Boot Record (MBR) or DOS boot sector found on hard disks and legacy external drives. If an infected USB flash drive is left plugged into a computer during startup, the device reads the corrupted boot sector first. The virus launches immediately before the main operating system even loads, allowing it to bypass early antimalware defenses.

12. Multipartite Virus

Multipartite viruses employ a multi-pronged strategy to maximize damage. They can simultaneously infect both your executable files and your hard drive's boot sector. This makes them exceptionally difficult to clean: if an IT administrator purges the program files but overlooks the boot sector, the virus will completely reinfect the machine the next time it boots up.

13. Polymorphic Virus

Polymorphic malware can dynamically mutate its underlying code signature every time it replicates, while maintaining its core malicious functionality. Because signature-based antivirus scanners rely on static database definitions to flag threats, changing its digital footprint allows a polymorphic virus to constantly evade detection.

Comparison: High-Risk Virus Categories