What Is Malware? Viruses, Worms, Trojans, Spyware, and How to Remove Them

What Is Malware?

Malware is an umbrella term for any software intentionally designed to cause disruption, damage data, steal information, gain unauthorized access, or extort money from individuals, organizations, or governments. The word is a portmanteau of malicious and software. Malware has existed almost as long as computers themselves — the first recognized computer virus, the Creeper program, appeared on ARPANET in the early 1970s — but the explosion of internet connectivity has made malware one of the defining challenges of the digital age.

Cybercriminals distribute malware through a variety of vectors: phishing emails with malicious attachments or links, compromised websites, infected USB drives, malvertising (malicious advertisements), software vulnerabilities, and increasingly, software supply chain attacks that embed malware in legitimate software updates or libraries. Understanding how each malware type operates is the first step toward defending against it.

Viruses

A computer virus is a type of malware that attaches itself to legitimate programs or files and replicates when the infected file is executed. Like a biological virus, a computer virus requires a host — it cannot spread on its own. When an infected program runs, the virus code executes and can infect other programs on the same system or on network-accessible drives. Classic viruses included ILOVEYOU (2000), which spread via email and caused an estimated $10 billion in damages, and Melissa (1999), which mass-mailed itself to Outlook address book contacts.

Modern viruses are less prevalent than they once were — they have largely been eclipsed by worms and trojans — but file infector viruses, macro viruses (targeting document macros in Word and Excel), and polymorphic viruses (which change their code signature to evade detection) remain active threats.

Worms

A worm is similar to a virus but does not require a host file — it is a standalone program that replicates itself and propagates across networks autonomously. Worms exploit vulnerabilities in operating systems or network services to spread without any user action. The WannaCry ransomware worm (2017), which exploited the NSA-developed EternalBlue vulnerability in Windows SMB, infected over 200,000 systems in 150 countries within days and caused billions of dollars in damages.

Worms can consume network bandwidth as they spread, degrade system performance, install backdoors for remote access, drop additional malware payloads, and form botnets — networks of compromised computers used for distributed attacks. Network segmentation, prompt patching, and firewalls that restrict unnecessary SMB, RDP, and other services are the primary defenses against worm propagation.

Trojans

A trojan horse (or trojan) disguises itself as a legitimate, useful program while secretly performing malicious functions. Unlike viruses and worms, trojans do not self-replicate; they rely on social engineering to trick users into installing them. Common trojan disguises include fake antivirus software, game cheats, media players, and productivity apps distributed through unofficial channels.

Trojans have diverse payloads: remote access trojans (RATs) give attackers persistent control over the victim's system; banking trojans intercept online banking sessions to steal credentials and redirect transactions; dropper trojans deliver additional malware to the infected system; and downloader trojans connect to a command-and-control server to retrieve additional payloads on demand.

Spyware and Adware

Spyware covertly monitors user activity and collects sensitive information — browsing history, login credentials, financial data, form inputs — and transmits it to a third party without the user's knowledge or consent. Spyware is often bundled with free software or installed through drive-by downloads. Commercial spyware products such as Pegasus, developed by NSO Group, have been used by governments to surveil journalists, activists, and political opponents, raising profound civil liberties concerns.

Adware is software that automatically displays or downloads advertising material, often generating revenue for its creators through fraudulent clicks. While adware is not always malicious in intent, aggressive adware can degrade system performance, redirect browser searches, and expose users to malvertising. The line between legitimate advertisement-supported software and malicious adware is often blurry.

Rootkits and Keyloggers

A rootkit is designed to conceal its presence and the presence of other malware from the operating system and security tools. Rootkits operate at the kernel level, modifying core operating system functions to hide files, processes, registry entries, and network connections. This deep integration makes rootkits extremely difficult to detect and remove — in some cases, reinstalling the operating system from clean media is the only reliable remediation. The Sony BMG copy protection rootkit scandal (2005), in which Sony secretly installed rootkit-like software on 22 million CDs, brought the concept into public awareness.

A keylogger records keystrokes and captures screenshots to collect passwords, credit card numbers, and other sensitive information. Keyloggers can be software-based (running as processes on the infected system) or hardware-based (physical devices plugged between the keyboard and computer). Software keyloggers are typically deployed as part of a larger malware package; hardware keyloggers require physical access and are used in targeted attacks against specific individuals or workstations.

Detection and Removal

Modern antivirus and endpoint detection and response (EDR) tools use multiple detection methods: signature-based detection (matching known malware code patterns), heuristic analysis (identifying suspicious code behaviors), behavioral monitoring (flagging anomalous process or network activity), and machine learning classifiers trained on large datasets of malware samples. No single method catches everything, which is why a layered defense is essential.

When malware is detected, remediation steps depend on the type and severity of infection. For most malware, running a reputable antimalware scanner in safe mode and removing identified threats is sufficient. For rootkits or persistent advanced threats, a full system wipe and reinstallation from clean backups may be necessary. After remediation, changing all passwords — especially those used during the infection period — resetting browser settings, and reviewing recent financial transactions are critical steps.

Prevention remains more effective than remediation. Key preventive practices include keeping operating systems and applications fully patched, using a reputable security suite, enabling multi-factor authentication, avoiding suspicious email attachments and links, downloading software only from official sources, and maintaining regular, offline backups to enable recovery from ransomware without paying the ransom.