What is a Computer Virus | Types of Computer Viruses | How They Affect Your Files (Data)

A computer virus is a type of malicious software (malware) designed to replicate itself and spread from one computer to another, often causing harm along the way. Much like a biological virus, a computer virus inserts its code into legitimate programs or files, then executes to alter, delete, or corrupt data, slow down systems, or open backdoors for other attacks. Understanding what viruses are, the different types that exist, and how they can affect your files is essential for protecting personal and business data in an increasingly connected world.

What Is a Computer Virus?

A computer virus is a self-replicating program that requires human action to spread such as opening an infected file, running a malicious program, or clicking a malicious link. Once active, a virus can attach itself to other files or system areas and perform malicious actions, which may be visible (e.g., pop-ups, file corruption) or stealthy (e.g., data theft, registry changes).

Common Characteristics of Viruses

Self-replication: The virus copies itself to other files or locations.
Activation method: It may activate on a trigger (specific date/time, event).
Payload: The harmful part that causes damage or unwanted behavior.
Concealment: Many viruses use obfuscation to evade detection.

Types of Computer Viruses

1. File Infector Viruses
Attach to executable files (.exe, .com). When the infected file runs, the virus runs too. Can corrupt or replace program code, causing crashes or altered behavior. Examples of file infector viruses include CIH, Sality, Virut, Ramnit, and Stoned.

2. Boot Sector Viruses

  Infect the Master Boot Record (MBR) or boot sector of storage media.
Launch during system startup and can prevent an OS from loading correctly. Examples of boot sector viruses include the classic Stoned and Michelangelo viruses

 

3. Macro Viruses
Exploit macros in office documents (Word, Excel).
Spread when documents are opened and macros are enabled, often through email attachments. Examples of macro viruses include the Melissa virus (1999), which used email to spread widely

 

4. Polymorphic Viruses
Change their code each time they replicate, making signature-based detection harder.
Use encryption and mutation engines to evade antivirus software. Examples of polymorphic viruses include the Storm Worm

 

5. Metamorphic Viruses
Rebuild their own code to avoid detection, more advanced than polymorphic types.
Can alter program structure while retaining functionality. Examples of metamorphic viruses include Win95/Zmist, released in 2000, and Win32/Simile (MetaPHOR)

 

6. Resident Viruses
Install themselves into system memory and remain active, intercepting system Operations. Can infect files even if the original infected file is no longer running.

 

7. Non-Resident Viruses
Do not stay in memory; they perform their actions and then terminate.
Often faster but less persistent than resident viruses.

 

8. Browser Hijackers and Web Malware
Modify browser settings, inject ads, or redirect traffic to malicious sites.
Often bundled with free software or through malicious scripts on web pages.

 

9. Ransomware (related malware)

Encrypts user files and demands payment for decryption keys.
While often considered separate from classic virus definitions, many ransomware strains spread like viruses and cause severe data loss.

 

10. Worms and Trojans (comparison)
Worms: Self-replicating malware that spreads across networks without needing to attach to files.
Trojans: Malicious programs disguised as legitimate software; they don’t self-replicate but often deliver harmful payloads.

 

How Viruses Affect Your Computer Files (Data)

 

1. File Corruption and Deletion
Viruses may overwrite or corrupt files, making them unusable or causing data loss.
Some viruses intentionally delete files to destroy evidence or cause damage.

2. Encryption and Ransomware
Ransomware encrypts files using strong cryptography and demands payment for the decryption key.
Even small businesses can be crippled if backups are not available.

3. Data Theft and Exfiltration
Some malware searches for sensitive files (financial records, credentials) and sends them to attackers.
Stolen data can lead to identity theft, financial fraud, or corporate espionage.

4. System Instability and Crashes
Infected system files or drivers can destabilize the OS, causing crashes, blue screens, or boot failure.
Boot sector viruses may prevent the system from starting at all.

5. Performance Degradation
Background replication and malicious processes consume CPU, memory, and network bandwidth.
Systems may become slow, unresponsive, or unable to run critical applications.

6. Unauthorized Access and Backdoors
 Some viruses install backdoors that allow attackers to control the system remotely.
 Backdoors enable additional malware installation, further data compromise, or use in botnets.

7. Propagation to Other Systems

Viruses often spread via removable media, shared networks, email attachments, or infected downloads risking broader infection in home or corporate networks.

How Infections Typically Occur

•   Opening infected email attachments or clicking malicious links.
•   Downloading pirated or cracked software from untrusted sources.
•   Inserting infected USB drives or removable media.
•   Visiting compromised websites or clicking pop-up ads.
•   Exploiting unpatched software vulnerabilities (drive-by downloads).

Detection and Signs of Infection

• Unexpected pop-ups, new toolbars, or home page changes in browsers.
• Slower system performance, high CPU or disk usage for no clear reason.
• Missing, corrupted, or suddenly inaccessible files.
• Unusual network activity or unknown programs running in Task Manager.
• Antivirus alerts or blocked actions

Prevention Best Practices

1. Keep Software Updated
  Regularly install OS and application security patches to close vulnerabilities.

2. Use Reputable Security Software
Install and maintain updated antivirus/antimalware solutions with real-time protection.

3. Practice Safe Browsing and Email Habits
Don’t open attachments or click links from unknown senders. Verify email senders and inspect URLs.
Avoid downloading software from unofficial or pirated sources.

4. Use Strong Authentication and Access Controls
Use strong, unique passwords, and enable multi-factor authentication where possible.

5. Backup Regularly and Verify Backups
Maintain offline or immutable backups to recover files if encryption or deletion occurs.

6. Disable Unnecessary Macros
Block or disable macros in Office documents unless they are from trusted sources.

7. Limit User Privileges
Grant users only the permissions they need; avoid running daily tasks with administrative rights.

8. Network Segmentation and Security
Isolate critical systems from general networks and use firewalls and intrusion detection/prevention systems.

Recovery and Response

1.      Isolate infected machines from the network immediately to prevent spread.

2.      Use offline backups to restore critical data after ensuring malware is removed.

3.      If ransomware is involved, consult cybersecurity professionals and law enforcement paying ransom is discouraged as it does not guarantee recovery.

4.      Reinstall OS from known-good sources if necessary after data recovery.

 

Computer viruses remain a persistent threat in modern computing environments. While their forms and tactics have evolved from simple file infectors to sophisticated ransomware and polymorphic threats the fundamentals of prevention remain constant: patch systems, use reliable security tools, practice safe behavior, and maintain verified backups. By understanding how viruses operate and how they affect your files, you can take strong, proactive steps to protect your data and keep systems secure.