Computer Viruses and Malware: Types, Examples, Symptoms and Protection

Computer viruses and malware can steal passwords, damage files, monitor users and stop entire computer systems from working.

They can affect:

Desktop computers
Laptops
Mobile phones
Servers
Websites
Routers
Smart cameras
Other internet-connected devices

Although people often use the words virus and malware as if they mean the same thing, there is an important difference between them.

What Is Malware?

Malware is the short form of malicious software.

It is software or code intentionally created to:

Steal information
Damage files
Spy on users
Take control of devices
Display unwanted advertisements
Demand money
Disrupt computer systems
Provide attackers with unauthorized access

CISA defines malware as software used to gain unauthorized access, steal data, disrupt services or damage IT systems. (CISA)

Simple Definition

Malware is any software intentionally designed to harm, spy on or misuse a computer, device or data.

Real-Life Example of Malware

Imagine someone secretly places a criminal inside a delivery box.

The box looks normal, so you bring it inside your house. After opening it, the criminal may:

Steal your belongings
Watch your activities
Damage your property
Lock your rooms
Open the door for other criminals

Malware works similarly. It may arrive inside a normal-looking application, document, attachment or download.

What Is a Computer Virus?

A computer virus is a type of malware that attaches itself to a file or program.

When the infected file is opened, the virus may execute and spread to other files or devices.

A virus may:

Modify files
Delete information
Corrupt programs
Slow down the computer
Display unwanted messages
Spread through USB drives
Download additional malware

Simple Definition

A computer virus is malware that attaches itself to a file or program and spreads when the infected item is executed.

Malware vs Computer Virus

All computer viruses are malware, but not all malware is a virus.

Consider this simple comparison:

Vehicle = Large category
Car = One type of vehicle

Malware = Large category
Virus = One type of malware

Malware	Computer Virus
General term for all malicious software	One particular type of malware
Includes ransomware, spyware, worms and Trojans	Usually attaches itself to a file or program
May spread in several different ways	Commonly spreads when infected code is executed
May or may not copy itself	Usually attempts to reproduce by infecting other files
Can target computers, phones, servers and smart devices	Traditionally associated with infected computer files

Main Types of Malware

flowchart TD
    A["Malware"]
    A --> B["Self-replicating malware"]
    A --> C["Disguised or hidden access"]
    A --> D["Data theft and spying"]
    A --> E["Disruption or financial gain"]
    B --> B1["Virus and worm"]
    C --> C1["Trojan, backdoor and rootkit"]
    D --> D1["Spyware, keylogger and infostealer"]
    E --> E1["Ransomware, botnet, wiper and cryptojacker"]

1. Computer Virus

A virus normally attaches itself to another file or application.

It becomes active when a user runs the infected file.

Real-Life Example

A student receives an infected presentation file through a USB drive. When the file is opened, malicious code runs and infects other files on the computer.

Common Virus Activities

A virus may:

Change file contents
Corrupt documents
Infect executable files
Copy itself to removable drives
Display messages
Disable security software

2. Computer Worm

A worm is malware that can copy itself and spread automatically across networks.

Unlike a traditional virus, a worm does not always need to attach itself to another file or wait for the user to open it.

A worm may exploit:

Unpatched software
Weak network services
Poorly secured devices
Stolen credentials

Real-Life Example

One unpatched office computer becomes infected. The worm searches the network for other vulnerable computers and spreads to them automatically.

Virus vs Worm

Virus	Worm
Usually attaches to a file or program	Usually works as a separate malicious program
Often requires an infected file to be opened	Can spread automatically
Commonly spreads through files or removable drives	Commonly spreads through networks
Infects other files	Infects other devices or systems

3. Trojan Horse

A Trojan is malware disguised as legitimate or useful software.

It is named after the ancient Trojan horse story, in which soldiers hid inside a wooden horse.

Trojan malware may pretend to be:

Free software
Game
PDF converter
Browser extension
Security tool
Software update
Mobile application

Real-Life Example

A user downloads a “free video editor” from an unknown website. The editor appears to work, but it secretly installs malware that steals browser passwords.

Important Point

A Trojan does not necessarily copy itself. It mainly depends on tricking the user into installing or opening it.

4. Ransomware

Ransomware is malware that blocks access to files or systems and demands payment.

Many ransomware attacks use encryption to make files unreadable.

An attacker may demand money in exchange for:

A decryption key
Restoring system access
Not publishing stolen data

Real-Life Example

A business employee opens a malicious attachment. The ransomware encrypts company documents and displays a message demanding cryptocurrency.

Double Extortion

Some ransomware attackers use two forms of pressure:

Encrypt the victim’s files.
Steal the data and threaten to publish it.

Paying a ransom does not guarantee that files will be recovered or stolen data will be deleted.

5. Spyware

Spyware secretly monitors a user and collects information.

It may collect:

Browsing activity
Login details
Personal messages
Location
Files
Screenshots
Microphone or camera information

Real-Life Example

Spyware is like a hidden camera placed inside someone’s room without permission.

The victim may continue using the computer without knowing that information is being collected.

6. Keylogger

A keylogger records the keys pressed on a keyboard.

It may capture:

Usernames
Passwords
Messages
Search queries
Payment details

Real-Life Example

Imagine someone secretly watching every key you press while entering your bank password. A software keylogger performs a similar activity digitally.

Some legitimate software can record keystrokes for authorized testing or accessibility purposes. It becomes malicious when used secretly without permission.

7. Information Stealer

An information stealer, or infostealer, is designed to collect valuable data quickly.

It may target:

Browser passwords
Cookies
Authentication tokens
Cryptocurrency wallets
Credit-card information
Email accounts
Documents
Screenshots

Stolen cookies or authentication tokens may sometimes help criminals access an account without entering the original password.

8. Adware

Adware displays excessive or unwanted advertisements.

It may:

Create pop-ups
Redirect the browser
Change the homepage
Install unwanted extensions
Track browsing behaviour

Not every advertisement is malware. Adware becomes a security concern when it is installed deceptively, collects information without proper permission or interferes with the device.

9. Potentially Unwanted Program

A Potentially Unwanted Program, or PUP, may not be clearly malicious but can still create problems.

It may:

Install additional applications
Change browser settings
Display advertisements
Collect unnecessary information
Slow down the device

PUPs are often included with free software installers.

10. Rootkit

A rootkit is designed to hide malicious activity and provide privileged access to a system.

A rootkit may hide:

Files
Processes
User accounts
Network connections
Other malware

Real-Life Example

A rootkit is like an intruder hiding inside the basement while also removing evidence that anyone entered the house.

Rootkits can be difficult to detect because they may operate at a deep level of the operating system.

11. Backdoor

A backdoor provides hidden access that bypasses normal authentication or security controls.

An attacker may use a backdoor to:

Return to the device later
Run commands
Install more malware
Steal files
Control the system remotely

Real-Life Example

A backdoor is like a secret entrance created in a building so that an intruder can return without using the main locked door.

12. Bot and Botnet Malware

A bot is an infected device controlled remotely by an attacker.

A collection of infected devices is called a botnet.

A botnet may contain:

Computers
Servers
Routers
Smart cameras
Digital video recorders
Other Internet of Things devices

Attackers may use botnets to:

Launch Distributed Denial-of-Service attacks
Send spam
Spread malware
Perform advertising fraud
Hide criminal traffic
Mine cryptocurrency

Real-Life Example

A botnet is like an army of controlled machines. The owners may not know their devices are being used in an attack.

13. Cryptojacking Malware

Cryptojacking malware secretly uses a victim’s computer resources to mine cryptocurrency.

Possible effects include:

Slow performance
High CPU usage
Excessive electricity use
Overheating
Reduced battery life
Hardware strain

Real-Life Example

It is like someone secretly connecting their equipment to your electricity supply and making you pay the bill.

14. Wiper Malware

Wiper malware is designed to destroy or erase data.

Unlike ransomware, its main purpose may be disruption or destruction rather than earning money.

A wiper may:

Delete files
Damage the operating system
Destroy boot information
Make devices unusable

15. Fileless Malware

Fileless malware performs much of its activity in memory or by misusing legitimate system tools.

It may leave fewer traditional files on storage, making some forms of detection more difficult.

The word “fileless” does not always mean that no files are involved at any stage. It mainly describes techniques that reduce dependence on ordinary malicious executable files.

How Does Malware Infect a Computer?

Malware commonly reaches devices through:

Phishing emails
Malicious attachments
Fake login pages
Unsafe websites
Pirated or cracked software
Fake updates
Unknown USB drives
Malicious advertisements
Infected mobile applications
Unpatched vulnerabilities
Compromised browser extensions
Software supply-chain attacks
Weak or stolen passwords

Typical Malware Infection Process

flowchart LR
    A["Malware is delivered"] --> B["File or exploit runs"]
    B --> C["Malware gains access"]
    C --> D["Malware hides or persists"]
    D --> E["Data theft, damage or remote control"]

Step 1: Delivery

The malicious content reaches the victim through an email, website, application, USB drive or network connection.

Step 2: Execution

The malware begins running because:

The user opens a file.
The user installs an application.
A malicious command is executed.
An attacker exploits a vulnerability.

Step 3: Access

The malware attempts to access files, accounts or system functions.

Step 4: Persistence

Some malware tries to remain active after the device restarts.

Step 5: Malicious Activity

The malware may:

Steal data
Encrypt files
Record keystrokes
Contact an attacker
Download additional malware
Spread to other devices
Delete information

Can a Computer Become Infected Without Clicking?

Yes.

Some attacks exploit security vulnerabilities and may not require an obvious click. Worms can also spread automatically between vulnerable systems.

However, many malware infections still depend on users:

Opening an attachment
Installing unknown software
Running a copied command
Disabling security warnings
Entering information on a fake page

Keeping software updated reduces the chance that known vulnerabilities can be exploited.

Common Signs of Malware Infection

Possible warning signs include:

Computer becomes unusually slow
Device frequently crashes
Unknown programs appear
Excessive pop-up advertisements
Browser homepage changes
Search results are redirected
Files disappear or become encrypted
Antivirus is disabled
New administrator accounts appear
Camera or microphone activates unexpectedly
Battery drains quickly
Device overheats
Unusual network activity
Messages are sent without permission
Bank or email accounts show unknown logins
Storage space suddenly decreases
Fans run continuously without a clear reason

Important Point

Some malware is designed to remain silent. A device may appear normal while an infostealer or spyware collects information.

Therefore, the absence of visible symptoms does not guarantee that a device is clean.

Real-World Malware Examples

WannaCry Ransomware and the NHS

On 12 May 2017, WannaCry spread worldwide by combining ransomware with worm-like spreading.

It encrypted information and demanded payment.

The attack became the largest cyberattack to affect the NHS in England at that time. The UK National Audit Office reported that:

At least 81 of 236 NHS trusts in England were affected.
603 primary-care and other NHS organizations were infected.
Thousands of appointments and operations were cancelled.
Many infected organizations were using unpatched or unsupported Windows systems.

The incident demonstrated how one malware outbreak can affect real-world healthcare services, not just computer files. (UK National Audit Office WannaCry Report)

Main Lesson

Install security patches, protect network services and prepare a tested incident-response plan.

GameOver Zeus Banking Malware

GameOver Zeus was designed to steal banking and other login credentials.

The FBI reported that:

More than one million computers were infected globally.
Infected devices became part of a botnet.
Criminals used stolen banking details to perform fraudulent transfers.
Estimated losses exceeded $100 million.

It mainly spread through spam and phishing messages. (FBI GameOver Zeus Report)

Main Lesson

A normal-looking email can lead to financial malware, account theft and a large botnet.

Mirai Botnet

Mirai targeted Internet of Things devices such as:

Routers
Wireless cameras
Digital video recorders

It infected poorly secured devices and turned them into remotely controlled bots.

The US Department of Justice reported that Mirai contained hundreds of thousands of compromised devices at its peak and was used for powerful Distributed Denial-of-Service attacks. (US Department of Justice)

Main Lesson

Smart devices also need strong passwords, updates and secure configuration.

Short Malware News Update

The following examples were current when this article was prepared in August 2026.

macOS ClickFix Infostealer Campaign

On 5 August 2026, Microsoft reported a macOS campaign using look-alike websites and fake download or verification instructions.

Victims were persuaded to copy and run commands in Terminal. The campaign then delivered information-stealing malware such as MacSync and Atomic Stealer.

Microsoft warned that a legitimate CAPTCHA, verification process or download should not require users to paste a command into Terminal. (Microsoft Security Research)

ChainDrop Software Supply-Chain Worm

On 4 August 2026, Microsoft reported a self-propagating credential-stealing worm in a large npm software supply-chain attack.

According to Microsoft, malicious releases affected more than 400 packages. The worm targeted developer, GitHub, cloud and infrastructure credentials and could use stolen publishing access to infect additional packages. (Microsoft Security Research)

Main Lesson from Recent News

Modern malware can target:

Windows computers
Mac computers
Mobile devices
Developers
Cloud accounts
Software packages
Smart devices

No operating system or device type should be considered completely immune.

How to Protect a Computer from Viruses and Malware

Keep Software Updated

Install updates for:

Operating system
Browser
Applications
Antivirus
Router
Mobile device

Updates often close vulnerabilities that malware could exploit.

Use Antivirus or Endpoint Protection

Keep real-time protection enabled and allow security software to update automatically.

Antivirus may:

Scan downloads
Block known malware
Monitor suspicious behaviour
Quarantine malicious files
Remove infections

Antivirus is useful, but it cannot replace safe behaviour, updates and backups.

Download from Trusted Sources

Use:

Official developer websites
Microsoft Store
Apple App Store
Google Play Store
Trusted software repositories

Avoid cracked, pirated or modified applications.

Check Email Attachments

Be careful with unexpected:

ZIP files
Office documents
PDF files
Executable files
Shortened links
Password-protected archives

Confirm unusual messages with the sender using another communication method.

Do Not Run Unknown Commands

A website may tell you to:

Paste something into PowerShell
Open Command Prompt
Run a Terminal command
Disable antivirus
Bypass a security warning

Do not follow these instructions unless you fully trust the source and understand the command.

Use Strong and Unique Passwords

Use a different password for every important account.

A password manager can help create and store long, unique passwords.

Enable Multi-Factor Authentication

MFA can reduce damage if a password is stolen.

Use stronger methods such as passkeys, authenticator applications or security keys when available.

Use a Firewall

A firewall helps control network connections between your device and other systems.

Keep the operating system and router firewalls enabled unless a trusted technical requirement says otherwise.

Back Up Important Files

Create backups of important:

Documents
Photographs
Website files
School assignments
Business data

Keep at least one backup separate from the main device. Regularly check that important files can actually be restored.

Use Standard User Accounts

Avoid using an administrator account for ordinary browsing and daily work.

Malware may cause more damage when it runs with administrator privileges.

Secure Smart Devices

For routers, cameras and other smart devices:

Change default passwords.
Install updates.
Disable unused remote access.
Remove unsupported devices.
Use secure Wi-Fi settings.

What to Do If Your Computer Has Malware

flowchart LR
    A["Disconnect"] --> B["Scan and investigate"]
    B --> C["Remove or rebuild"]
    C --> D["Change passwords"]
    D --> E["Restore and monitor"]

1. Disconnect the Device

Disconnect Wi-Fi, Ethernet and unnecessary external drives if the malware may be stealing information or spreading.

Do not disconnect a business device without following your organization’s incident-response instructions.

2. Stop Sensitive Activity

Do not use the suspected device for:

Online banking
Shopping
Password changes
Important email
Cryptocurrency accounts

Use a known-clean device instead.

3. Inform the Right Person

If it is a school or workplace device, contact the IT or security team immediately.

Do not try to hide the incident.

4. Update and Scan

Use trusted, updated security software to perform a full scan.

For persistent Windows infections, Microsoft provides an offline scanning option through Windows Security. (Microsoft Windows Security)

5. Follow the Security Tool’s Advice

Detected malware may need to be:

Quarantined
Removed
Blocked

Do not restore a detected file simply because you recognize its filename. Malware can use misleading names.

6. Change Passwords from a Clean Device

Change passwords for affected accounts, starting with email.

Also:

Enable MFA.
Sign out of other sessions.
Check email-forwarding rules.
Review account recovery details.
Change reused passwords.

7. Restore or Reinstall If Necessary

If trusted security software cannot remove the infection, the safest option may be to erase the device and reinstall its operating system.

Restore files only from a backup believed to be clean.

The UK NCSC recommends updating the device, running an antivirus scan and following its advice. If the infection cannot be cleaned, reinstalling the system or getting expert assistance may be required. (NCSC Infected Device Guidance)

8. Check Other Devices

If the infected device was connected to a home, school or office network, check other connected devices for unusual activity.

9. Contact Financial Providers

If banking or payment details may have been stolen, contact the bank or payment provider through its official contact information.

What Not to Do

Do not keep using an infected device normally.
Do not enter new passwords on it.
Do not install random “cleaner” applications.
Do not pay someone who contacts you unexpectedly.
Do not restore malware from quarantine without investigation.
Do not connect possibly infected USB drives to other computers.
Do not assume that deleting one suspicious file removed the complete infection.
Do not test real malware on your personal computer.

Trusted External Resources

For updated information and recovery guidance, readers can use:

Common Misconceptions

Misconception 1: Every Malware Program Is a Virus

A virus is only one type of malware.

Misconception 2: Mac and Linux Devices Cannot Get Malware

Every widely used operating system can be targeted. The malware types and frequency may differ, but no platform is completely immune.

Misconception 3: Antivirus Provides Complete Protection

Antivirus is only one security layer. Updates, backups, MFA and safe behaviour are also required.

Misconception 4: Only Executable Files Can Contain Malware

Malicious activity can also arrive through documents, scripts, browser extensions, software packages and exploited vulnerabilities.

Misconception 5: A Fast Computer Cannot Be Infected

Some malware uses very few resources and tries to remain hidden.

Misconception 6: A Factory Reset Always Solves Every Security Problem

A reset can remove many infections, but accounts, cloud data, routers or backups may also be compromised.

Misconception 7: A Familiar Filename Means the File Is Safe

Attackers frequently use trustworthy-looking names and icons.

Frequently Asked Questions

What is malware in simple words?

Malware is software intentionally created to harm, spy on or misuse a computer or its data.

What is a computer virus?

A virus is malware that attaches itself to a file or program and spreads when the infected code is executed.

Is a virus different from malware?

Yes. Malware is the complete category, while a virus is one type of malware.

What is the most dangerous type of malware?

There is no single most dangerous type. The damage depends on the malware, target and security controls. Ransomware, infostealers, spyware, rootkits and wipers can all cause serious harm.

Can malware steal passwords?

Yes. Keyloggers, spyware and infostealers can steal passwords, cookies and authentication tokens.

Can a phone get malware?

Yes. Mobile devices can be infected through harmful applications, messages, websites and unpatched vulnerabilities.

Can malware spread through Wi-Fi?

Malware does not infect a device merely because Wi-Fi exists. However, worms and attackers can use a poorly secured network to find and attack vulnerable devices.

Can a PDF or Word file contain malware?

A document can contain malicious links, scripts, embedded content or exploit code. Treat unexpected documents carefully.

Does deleting an infected file remove the malware?

Not always. The malware may have created other files, changed settings or added persistence.

Should I pay a ransomware demand?

Payment does not guarantee recovery or deletion of stolen data. Individuals and organizations should contact relevant professionals and authorities and follow an established response plan.

Is free antivirus enough?

Built-in or free security tools can provide useful protection when supported, enabled and updated. Protection quality depends on the product and configuration, and safe behaviour remains essential.

How can I know whether malware has been removed?

Run updated security scans, review accounts and device behaviour, and follow trusted recovery guidance. For serious or persistent infections, rebuilding the device from a trusted source may be safer.

Conclusion

Malware is the broad term for malicious software, while a computer virus is one specific type of malware.

The easiest way to remember the difference is:

Every virus is malware, but every malware program is not a virus.

Malware can spread through phishing, unsafe downloads, software vulnerabilities, compromised applications and poorly secured devices.

The main protection steps are:

Update → Scan → Think Before Clicking → Use MFA → Back Up Data

If malware is detected, disconnect the affected device, use trusted security tools, change passwords from a clean device and restore only from clean backups.