Computer viruses are malicious programs designed to disrupt, damage, or gain unauthorized access to computer systems. Just like biological viruses, they replicate and spread from one system to another—causing data loss, system crashes, or even identity theft.
in other words, A computer virus is a type of malicious software program designed to interfere with the normal operation of a computer. It can replicate itself and spread from one file or system to another, often without the user’s knowledge. Like biological viruses, they “infect” and can damage systems or data.
Viruses spread through email attachments, downloaded software, infected websites, USB drives, or network connections. Users often unknowingly activate them by opening files or clicking malicious links. Once active, the virus can replicate and move across other files and devices.
There are many types of viruses, including file infector viruses, macro viruses, boot sector viruses, worms, and ransomware. Each type has a unique method of infection and effect on the system. Some steal data, others corrupt files, and some can even lock systems until a ransom is paid.
Common signs of a virus include slow computer performance, frequent crashes, unexpected pop-ups, missing files, or unusual network activity. Some viruses are silent and hard to detect, while others show immediate symptoms once the system is infected.
Preventing viruses involves using reliable antivirus software, keeping the operating system and applications updated, and avoiding suspicious links or downloads. Users should also regularly back up important files and disable unnecessary macros or scripts in documents.
Computer Viruses
File Infector Virus
A file infector virus attaches itself to executable files such as .exe or .com. When the infected file is run, the virus activates and may corrupt or overwrite other files on the system. It can spread quickly through shared drives and removable devices.
How to Avoid:
- Avoid downloading software from untrusted sources.
- Use reliable antivirus software that scans new files automatically.
- Regularly back up important files.
Macro Virus
Macro viruses target applications like Microsoft Word or Excel that use macro programming. When you open an infected document, the virus runs automatically and can modify or delete data.
How to Avoid:
- Disable macros in Office applications unless absolutely necessary.
- Do not open email attachments from unknown senders.
- Use an up-to-date office suite with built-in macro protection.
Boot Sector Virus
This type of virus infects the master boot record (MBR) of a storage device. It loads into memory during system startup, making it difficult to detect or remove. These viruses were common on floppy disks but can still spread via USB drives or infected bootable media.
How to Avoid:
- Do not use unverified or pirated USB drives and CDs.
- Enable BIOS protection settings against boot sector changes.
- Keep your operating system updated.
Polymorphic Virus
A polymorphic virus changes its code each time it replicates, making it difficult for antivirus software to detect it using standard signatures. It can infect files, emails, or network systems while staying hidden.
How to Avoid:
- Use advanced antivirus programs that use heuristic and behavior-based detection.
- Keep antivirus definitions updated daily.
- Avoid suspicious downloads or links.
Resident Virus
Resident viruses hide in a computer’s memory and activate when the system runs certain functions or programs. They can interfere with system operations, slow performance, or corrupt files.
How to Avoid:
- Perform regular full-system scans.
- Avoid running unknown executables.
- Keep your system’s startup programs clean and monitored.
Multipartite Virus
This is a hybrid virus that attacks both the boot sector and executable files, spreading in multiple ways. It can infect a system even after one part of it has been removed, making it especially dangerous.
How to Avoid:
- Run complete virus scans regularly.
- Avoid inserting external drives into infected systems.
- Maintain regular system backups.
Web Scripting Virus
Web scripting viruses are embedded in websites or pop-up ads using malicious code (such as JavaScript). When you visit the infected website, the virus can steal cookies, login data, or redirect you to harmful pages.
How to Avoid:
- Use a secure web browser with anti-phishing and script-blocking features.
- Avoid clicking suspicious ads or pop-ups.
- Keep browser extensions and security settings up to date.
Worm
Unlike most viruses, worms don’t need a host file—they can replicate and spread across networks independently. Worms consume bandwidth, slow down systems, and can install backdoors for hackers.
How to Avoid:
- Use a strong firewall to block unauthorized network access.
- Keep operating systems and applications patched.
- Avoid opening links from unverified emails or instant messages.
Browser Hijacker
A browser hijacker modifies your web browser’s settings without permission. It can change your homepage, redirect search queries to unwanted sites, or inject ads into pages. These viruses are often bundled with free software or extensions.
How to Avoid:
- Download browser extensions only from trusted sources.
- Use reputable anti-malware tools to scan for hijackers.
- Always choose “Custom” installation to deselect unwanted bundled programs.
Overwrite Virus
An overwrite virus deletes or replaces the content of infected files with its own malicious code. Once it attacks a file, the original data is lost permanently. It spreads through email attachments and infected downloads.
How to Avoid:
- Backup important files frequently to external or cloud storage.
- Do not open unknown email attachments.
- Use antivirus software with real-time protection to detect and remove threats early.
Network Virus
Network viruses spread rapidly through local networks and the internet. They exploit vulnerabilities in network protocols and can infect multiple connected systems, leading to slowdowns or total network failure.
How to Avoid:
- Keep all networked systems patched and updated.
- Use a firewall to block suspicious connections.
- Regularly monitor network traffic for unusual activity.
Companion Virus
A companion virus creates a duplicate of an existing program, usually by using the same name but a different file extension (for example, program.exe and program.com). When you try to open the original file, the infected version runs instead.
How to Avoid:
- Show file extensions in your system settings to spot suspicious duplicates.
- Run periodic system scans with updated antivirus software.
- Be cautious when using shared drives or unknown USB devices.
Direct Action Virus
This virus remains dormant until the infected file is executed. Once triggered, it replicates itself and infects other programs in the same directory or drive. It doesn’t usually alter files but can slow down performance.
How to Avoid:
- Perform scheduled virus scans weekly.
- Avoid pirated or cracked software downloads.
- Use antivirus software that includes heuristic analysis.
Logic Bomb (Time Bomb)
A logic bomb is a piece of malicious code hidden inside a legitimate program. It activates only when specific conditions are met — such as a certain date, time, or system event — often deleting files or crashing the system.
How to Avoid:
- Avoid installing unverified or pirated software.
- Monitor system files for unexpected changes.
- Use endpoint protection tools that detect unusual activity.
Ransomware
Ransomware locks or encrypts your data and demands payment (a ransom) to restore access. It spreads through phishing emails, infected websites, or software vulnerabilities. It is one of the most dangerous and costly types of malware today.
How to Avoid:
- Regularly back up data to an external drive or secure cloud storage.
- Do not click on suspicious links or attachments in emails.
- Keep your antivirus, operating system, and browsers fully updated.
- Consider using anti-ransomware tools for additional protection.
Stealth Virus
A stealth virus is designed to conceal its presence by intercepting system requests and returning false information. It hides its file size, location, or code to prevent detection by antivirus programs. These viruses often infect system files and remain dormant until activated.
How to Avoid:
- Use advanced antivirus software with heuristic and behavior-based scanning.
- Perform scans in Safe Mode to detect hidden threats.
- Keep your operating system and antivirus definitions updated regularly.
Email Virus
Email viruses spread through infected attachments, links, or scripts embedded in emails. Once opened, they can steal contacts, send spam, or install other forms of malware on the victim’s system. They often use social engineering tactics to trick users.
How to Avoid:
- Never open attachments or links from unknown or suspicious senders.
- Use an email service with built-in spam and virus filtering.
- Educate yourself on phishing techniques to recognize fake emails.
Spacefiller (Cavity) Virus
A spacefiller virus inserts malicious code into the empty spaces within legitimate files, without increasing the file’s size. This makes detection extremely difficult. When the infected program runs, the virus activates and may spread to other files or drives.
How to Avoid:
- Regularly scan your system with deep-level antivirus tools.
- Avoid using untrusted external storage devices.
- Keep backups of important programs and files to restore clean versions if needed.
Rootkit Virus
Rootkit viruses provide hackers with administrator-level access to a computer, allowing them to control the system, steal data, or hide other malware. They are extremely difficult to detect because they integrate deeply into the operating system.
How to Avoid:
- Use anti-rootkit tools and security software that scan at the kernel level.
- Avoid downloading or installing software from unverified sources.
- Keep your OS and security patches updated to fix known vulnerabilities.
Keylogger Virus
A keylogger virus records every keystroke made on a computer, allowing cybercriminals to capture sensitive information like passwords, credit card details, and personal messages. Keyloggers can be part of spyware or Trojan attacks.
How to Avoid:
- Use antivirus software with anti-keylogging features.
- Avoid typing sensitive information on public or shared computers.
- Regularly monitor accounts for suspicious login attempts or transactions.
Trojan Horse Virus
Named after the ancient Greek tale, a Trojan horse virus disguises itself as legitimate software to trick users into installing it. Once inside, it opens a backdoor for hackers to steal information, install more malware, or gain full system access. Unlike traditional viruses, Trojans don’t replicate on their own.
How to Avoid:
- Download software only from trusted, verified websites.
- Use a firewall to block unauthorized network activity.
- Avoid clicking pop-ups or “free download” ads online.
- Keep real-time protection enabled in your antivirus software.
Adware
Adware is a type of malware that automatically displays or downloads advertising material, often without user consent. It can slow down your computer, redirect browser searches, and track your browsing behavior to show targeted ads. While not always harmful, some adware collects personal information or installs additional unwanted programs.
How to Avoid:
- Always choose custom installation when downloading free software to deselect bundled adware.
- Use a reputable ad-blocker and anti-adware tool.
- Avoid clicking on pop-up ads or suspicious banners.
- Regularly clean your browser extensions and temporary files.
Spyware
Spyware secretly monitors your activities, such as keystrokes, browsing history, and login credentials, then sends that data to a remote attacker. It often comes bundled with free software or as part of phishing scams. Spyware can slow system performance and compromise privacy.
How to Avoid:
- Install a comprehensive anti-spyware program.
- Avoid downloading software from unverified or suspicious websites.
- Regularly check for and remove unfamiliar programs from your computer.
- Enable firewall and browser privacy settings.
Mobile Virus
A mobile virus targets smartphones and tablets, spreading through malicious apps, text messages, or compromised websites. It can steal data, send unauthorized messages, or corrupt system files. As mobile devices become more connected, these viruses are increasingly common.
How to Avoid:
- Download apps only from official app stores like Google Play or Apple App Store.
- Keep your mobile OS and apps updated.
- Do not click on suspicious links in texts or emails.
- Install mobile antivirus software for extra protection.
Cryptojacking Virus
Cryptojacking viruses secretly use your computer’s processing power to mine cryptocurrency for cybercriminals. While they don’t always damage files, they slow your computer, overheat hardware, and increase electricity usage.
How to Avoid:
- Use browser extensions that block mining scripts.
- Avoid visiting suspicious websites or clicking random links.
- Monitor your computer’s CPU usage for unexplained spikes.
- Keep antivirus and browser security settings active.
Network Worm (Internet Worm)
A network worm is a self-replicating program that spreads across computers via the internet or local networks, often without human interaction. It exploits security flaws to propagate and can deliver payloads such as spyware or ransomware.
How to Avoid:
- Keep all systems and software patched with the latest updates.
- Use a firewall to restrict unauthorized traffic.
- Scan your network regularly for suspicious connections.
- Disable unused network ports and services.
Browser Redirect Virus
Also known as a “search redirect” virus, this malware changes your browser settings and search engine preferences, sending you to fake or unsafe websites. It often installs itself through malicious downloads or bundled software.
How to Avoid:
- Be careful during installations—uncheck any optional software or toolbars.
- Use browser security extensions that detect redirects.
- Reset your browser settings if you notice unusual redirects.
- Perform full-system malware scans if your searches are hijacked.
Fake Antivirus (Rogue Security Software)
A fake antivirus virus pretends to be legitimate security software, warning you of false threats to trick you into paying for fake protection. Once installed, it can disable real antivirus software and steal credit card information.
How to Avoid:
- Never trust pop-up warnings claiming your system is infected.
- Only download antivirus software from official vendor websites.
- Use real-time monitoring to block fake programs.
- Keep your actual antivirus program updated and active.
Network Boot Virus
A network boot virus infects the system through the network’s boot process rather than a local disk. When a computer starts up using a shared network drive or remote boot server, this virus can load itself into memory and spread to other connected devices. It primarily targets organizations or systems with centralized network boot configurations.
How to Avoid:
- Protect network boot servers with strong authentication and restricted access.
- Keep network firmware and software updated.
- Scan all network drives for infections regularly.
- Disable network boot features if not in use.
Macro Worm
A macro worm spreads using macro-enabled files, such as Microsoft Word or Excel documents. Unlike a standard macro virus, it can automatically send infected documents to contacts via email or shared folders, spreading rapidly in office environments.
How to Avoid:
- Disable macros by default in your Office programs.
- Use email filters that detect and block macro-enabled attachments.
- Do not open unexpected Office documents, even from familiar contacts.
- Regularly update your Office suite to patch vulnerabilities.
Web Browser Exploit Virus
This type of virus takes advantage of security loopholes or vulnerabilities in web browsers and plugins (like Flash, Java, or outdated extensions). When users visit a compromised website, the virus installs itself silently, often leading to data theft or further malware downloads.
How to Avoid:
- Keep your web browser and plugins updated at all times.
- Avoid visiting unsecured websites (look for HTTPS).
- Use a browser security extension or sandbox environment.
- Disable unnecessary plugins and enable automatic updates.
Fileless Malware
Fileless malware operates entirely within a computer’s memory rather than being stored on disk. This makes it very difficult to detect using traditional antivirus methods. It exploits legitimate system tools like PowerShell or Windows Management Instrumentation (WMI) to perform malicious actions such as data theft or installing backdoors.
How to Avoid:
- Use behavior-based antivirus software that detects unusual activity, not just files.
- Keep your system tools and scripts locked down or restricted.
- Disable or limit the use of PowerShell on non-administrative accounts.
- Regularly monitor your system for unusual processes or spikes in resource usage.
General Tips to Protect Against Computer Viruses
- Install a trusted antivirus program and keep it updated.
- Regularly update your operating system and all installed software.
- Back up your data frequently to an external drive or cloud storage.
- Be cautious with emails, links, and attachments from unknown sources.
- Use strong passwords and enable multi-factor authentication where possible.