Understanding the Types of Computer Viruses

In today’s digital age, computer viruses remain a significant threat to cybersecurity, affecting millions of devices worldwide. These malicious programs can cause extensive damage, ranging from minor annoyances to severe data loss and financial harm. Understanding the various types of computer viruses is crucial for developing effective strategies to protect your systems. Here, we’ll explore the main categories of computer viruses, their characteristics, and how they operate.

1. File Infector Viruses

File infector viruses are among the most common types of computer viruses. They attach themselves to executable files (.exe) and spread when these files are run. Once activated, these viruses can overwrite or modify files, leading to program malfunctions or data loss. Examples include the CIH virus and the Sasser virus.


  • Attach to executable files.
  • Activate when the infected file is run.
  • Can cause program malfunctions and data loss.

2. Macro Viruses

Macro viruses target software applications that use macros, such as Microsoft Office programs (Word, Excel). These viruses are written in the macro language of the targeted application and are spread primarily through infected documents and email attachments. The Melissa virus is a well-known example.


  • Written in macro languages (e.g., VBA for Microsoft Office).
  • Spread through infected documents and email attachments.
  • Often cause disruptions in document processing.

3. Boot Sector Viruses

Boot sector viruses infect the master boot record (MBR) of a hard drive. These viruses are particularly insidious because they load before the operating system during startup, making them difficult to detect and remove. The Michelangelo virus is a notorious example.


  • Infect the master boot record.
  • Activate during the boot process.
  • Difficult to detect and remove.

4. Multipartite Viruses

Multipartite viruses are hybrid viruses that can infect both files and the boot sector. They combine characteristics of file infector and boot sector viruses, making them versatile and challenging to eradicate. The Tequila virus exemplifies this type.


  • Can infect both files and boot sectors.
  • Versatile and difficult to remove.
  • Spread through multiple vectors.

5. Polymorphic Viruses

Polymorphic viruses are designed to evade detection by changing their code each time they infect a new system. This ability to mutate makes them particularly difficult for traditional antivirus programs to identify and eliminate. The Storm Worm is a well-known polymorphic virus.


  • Change their code to avoid detection.
  • Difficult for traditional antivirus programs to detect.
  • Spread through various methods, including email and downloads.

6. Resident Viruses

Resident viruses embed themselves in a computer’s memory, allowing them to execute whenever the operating system or specific applications are run. These viruses can be particularly damaging because they can evade detection and removal while the system is running. The Randex and CMJ viruses are examples.


  • Embed in system memory.
  • Execute when the operating system or specific applications run.
  • Difficult to detect while the system is running.

7. Non-Resident Viruses

Non-resident viruses do not embed themselves in system memory. Instead, they remain within the infected file and activate only when the file is executed. Although they are generally easier to detect and remove, they can still cause significant damage. The Vienna virus is an example.


  • Do not embed in system memory.
  • Activate only when the infected file is run.
  • Easier to detect and remove than resident viruses.

8. Rootkit Viruses

Rootkit viruses are designed to gain administrative control over a system while hiding their presence. They can modify system files and create backdoors for other malware. Rootkits are particularly dangerous because they can remain undetected for long periods. Examples include the TDSS and Alureon rootkits.


  • Gain administrative control over systems.
  • Hide their presence.
  • Can modify system files and create backdoors.

9. Trojan Horses

Although technically not viruses, Trojan horses are often grouped with them due to their similar behavior. Trojans disguise themselves as legitimate software to trick users into installing them. Once installed, they can create backdoors, steal data, or install other malware. Notable examples include the Zeus and Emotet Trojans.


  • Disguise as legitimate software.
  • Trick users into installing them.
  • Can create backdoors, steal data, or install other malware.


Understanding the different types of computer viruses is essential for developing effective cybersecurity strategies. Each type of virus has unique characteristics and methods of infection, requiring tailored approaches to detection and removal. By staying informed and vigilant, users can better protect their systems from these pervasive threats. Regular updates, robust antivirus software, and safe browsing practices are critical components in defending against computer viruses.

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