What Is Malware?

Malware -- short for malicious software -- is any program or code designed to harm, exploit, or otherwise compromise a computer system without the user's consent. Malware is an umbrella term that covers many distinct types of threats, each with different behaviors, delivery methods, and objectives.

Modern malware ranges from simple nuisance programs that display unwanted advertisements to sophisticated tools used by nation-state actors for espionage and sabotage. Understanding the different categories helps you recognize threats, respond appropriately, and choose effective defenses.

Viruses, Worms, and Trojans

Viruses

A virus is malware that attaches itself to a legitimate program or file and spreads when that file is executed or shared. Like a biological virus, it requires a host to propagate. Viruses cannot spread on their own -- they depend on human action such as opening an infected email attachment, running a compromised program, or sharing an infected USB drive.

Viruses can corrupt or delete files, slow down your system, or serve as a delivery mechanism for other malware. Some viruses are polymorphic, meaning they change their code each time they replicate to evade antivirus detection.

Worms

Unlike viruses, worms are self-replicating malware that spread across networks without requiring human interaction. A worm exploits vulnerabilities in operating systems or network protocols to copy itself from one machine to another automatically. This makes worms particularly dangerous -- a single infection can spread to thousands of machines within hours.

The WannaCry attack of 2017 is a notable example. It exploited a vulnerability in Windows SMB protocol to spread across networks automatically, encrypting files on over 200,000 computers in 150 countries within a single day.

Trojans

A Trojan (named after the Trojan Horse of Greek mythology) disguises itself as legitimate software to trick users into installing it. Unlike viruses and worms, Trojans do not replicate themselves. Instead, they rely on social engineering -- the user downloads what they believe is a useful program, game, or utility, but it contains hidden malicious code.

Once installed, Trojans can create backdoors for remote access, steal credentials, download additional malware, or turn your computer into part of a botnet. Remote Access Trojans (RATs) are particularly dangerous because they give attackers full control over the infected system.

Spyware and Adware

Spyware

Spyware secretly monitors your activity and sends the collected information to a third party. It can track your browsing history, capture screenshots, record keystrokes, access your webcam and microphone, and harvest passwords and financial data. Commercial spyware (sometimes called "stalkerware") is also used for domestic surveillance, which is both unethical and illegal in most jurisdictions.

Advanced spyware like Pegasus, developed by the NSO Group, can infect smartphones with zero-click exploits -- meaning no user interaction is needed at all. While tools like Pegasus target high-profile individuals, consumer-grade spyware is widely available and commonly used in harassment and identity theft.

Adware

Adware displays unwanted advertisements on your device, often through browser pop-ups, injected ads on web pages, or system notification spam. While adware is generally less dangerous than other malware types, it degrades your computing experience, slows down your system, and may track your browsing habits to serve targeted ads. Some adware also serves as a gateway for more dangerous malware by redirecting you to malicious websites.

Rootkits and Keyloggers

Rootkits

A rootkit is malware designed to gain and maintain privileged access to a system while hiding its presence from the operating system and security software. Rootkits operate at a very low level -- sometimes embedding themselves in the operating system kernel, the boot sector, or even the firmware of hardware components.

Because rootkits conceal themselves from the OS itself, they are extremely difficult to detect with conventional antivirus tools. Detecting a rootkit often requires booting from external media and scanning the drive offline, or using specialized rootkit detection tools that compare expected system behavior with actual system behavior.

Keyloggers

A keylogger records every keystroke you make and transmits the log to an attacker. This captures passwords, credit card numbers, private messages, and anything else you type. Keyloggers can be software-based (installed as malware) or hardware-based (a small device plugged between your keyboard and computer).

What a keylogger captures:

[2026-03-03 10:15:22] bankofamerica.com
[2026-03-03 10:15:25] TAB john.doe@email.com
[2026-03-03 10:15:28] TAB MyS3cretP@ssw0rd!      <-- Your password, in plaintext
[2026-03-03 10:15:30] ENTER

How Malware Spreads

Understanding malware delivery methods helps you avoid the most common infection vectors. Attackers use a variety of techniques to get malware onto your system.

• Email attachments -- Infected documents, spreadsheets, or executable files sent as email attachments remain the most common delivery method
• Malicious downloads -- Software from untrusted sources, pirated content, and fake "free" tools frequently bundle malware
• Drive-by downloads -- Visiting a compromised or malicious website can trigger automatic malware downloads without your knowledge, exploiting browser vulnerabilities
• USB drives and removable media -- Infected USB drives can execute malware automatically when plugged in, especially on systems with autorun enabled
• Network propagation -- Worms and some other malware types spread automatically across networks by exploiting unpatched vulnerabilities
• Software supply chain attacks -- Attackers compromise legitimate software update mechanisms to distribute malware to all users of that software
• Malicious ads (malvertising) -- Legitimate advertising networks can unknowingly serve ads containing malicious code

Signs of Infection and Prevention

Signs Your Device May Be Infected

• Significant slowdown -- Your system is noticeably slower than normal, especially during startup
• Unexpected pop-ups -- Advertisements or warning messages appearing outside of your browser
• Unknown programs -- Applications you do not remember installing appear in your program list or taskbar
• High network activity -- Your internet connection is unusually slow or your data usage spikes for no apparent reason
• Disabled security tools -- Your antivirus or firewall has been turned off and you cannot re-enable it
• Unusual account activity -- Emails sent from your account that you did not write, or login attempts you did not make
• Files changed or missing -- Documents are encrypted, renamed, or deleted without your action

Prevention Best Practices

• Keep everything updated -- Install operating system, browser, and application updates promptly. Most malware exploits known vulnerabilities that patches have already fixed
• Use reputable antivirus software -- Keep it running and up to date. Enable real-time scanning
• Download from official sources only -- Use official app stores, developer websites, and verified package repositories
• Be cautious with email attachments -- Do not open attachments from unknown senders, and verify unexpected attachments even from known contacts
• Disable autorun for removable media -- Prevent USB drives from executing code automatically when plugged in
• Use a standard user account -- Run as a standard user rather than administrator for daily tasks. This limits what malware can do if it executes
• Enable your firewall -- A properly configured firewall blocks unauthorized network connections
• Back up your data regularly -- Backups do not prevent infection, but they ensure you can recover without paying ransoms or losing files permanently

What to Do If Infected

If you suspect your device is infected with malware, take immediate action to contain the damage and begin the cleanup process.

• Disconnect from the network -- Unplug your Ethernet cable or disable Wi-Fi to prevent the malware from spreading to other devices or communicating with its command server
• Do not log in to sensitive accounts -- If a keylogger is active, any credentials you type will be captured
• Boot into safe mode -- Safe mode loads only essential system services, which may prevent the malware from running
• Run a full antivirus scan -- Use an updated antivirus tool. Consider using a bootable antivirus rescue disk for thorough scanning
• Remove detected threats -- Follow your antivirus tool's recommendations for quarantine or removal
• Change all passwords -- After cleaning the infection, change passwords for all accounts accessed from the infected device, using a different clean device
• Monitor your accounts -- Watch for unauthorized activity on financial accounts, email, and social media for several weeks after the incident
• Consider a clean reinstall -- For serious infections, especially rootkits, a complete OS reinstall from clean media is the most reliable way to ensure the malware is fully removed

Summary

Malware is a broad category of threats that includes many distinct types, each with different behaviors and risks. Understanding what you are facing helps you respond effectively.

• Viruses attach to files and require human action to spread
• Worms self-replicate across networks without user interaction
• Trojans disguise themselves as legitimate software to trick you into installing them
• Spyware secretly monitors your activity and steals personal data
• Rootkits hide deep in your system to maintain persistent, undetectable access
• Keyloggers capture everything you type, including passwords and financial information
• Prevention requires keeping software updated, using antivirus, downloading from official sources, and practicing caution with email and removable media
• If infected, disconnect from the network, scan thoroughly, change passwords from a clean device, and consider a full reinstall for severe infections