A computer virus is a type of malicious software (malware) designed to spread from one computer to another by attaching itself to other programs or files, and then replicating itself when those programs are executed or files are opened. The main objective of a computer virus is to disrupt normal system operations, cause damage to data or software, and sometimes steal information or allow unauthorized access.

Computer viruses typically require some user action to spread, such as opening an infected email attachment, downloading and running a malicious file, or visiting a compromised website. Once activated, a virus can corrupt, delete, or encrypt files, slow down system performance, or even render a computer unusable. Unlike computer worms, which can spread independently without user interaction, viruses usually rely on the execution of an infected host file.

There are many types of computer viruses, including boot sector viruses, file infectors, macro viruses, and polymorphic viruses, each with different methods of infection and impact. Protecting against computer viruses involves using reputable antivirus software, keeping software updated, avoiding suspicious downloads or email attachments, and practicing safe browsing habits.

Notable Computer Viruses

Mydoom (2004)
• Widely regarded as the most destructive computer virus ever, Mydoom spread rapidly via email and targeted major technology companies with DDoS attacks. It caused an estimated $38 billion in damages and is still active today.
ILOVEYOU (2000)
• This worm disguised itself as a love letter in email attachments. Once opened, it overwrote files and sent copies of itself to all contacts in the victim’s address book. It infected millions of computers worldwide, causing up to $15 billion in damages.
Melissa (1999)
• Delivered via a Microsoft Word document, Melissa spread by emailing itself to the first 50 contacts in a user’s Outlook address book. It overwhelmed email servers globally and caused at least $80 million in damages.
Klez (2001)
• A polymorphic worm that spoofed email addresses and was hard to detect. It infected millions of computers and caused nearly $20 billion in damages.
Conficker (2008)
• This worm exploited Windows vulnerabilities to create a global botnet, infecting millions of systems, including those in government and military networks. Its estimated damage is over $9 billion.
Slammer (SQL Slammer, 2003)
• A fast-spreading worm that targeted Microsoft SQL Server, causing internet slowdowns and outages in critical infrastructure. Damages were estimated at over $1 billion.
Code Red (2001)
• Exploited vulnerabilities in Microsoft’s web server software, defacing websites and launching DDoS attacks, including one on the White House. It infected nearly a million hosts and caused over $2 billion in damages.
Sobig (2003)
• A family of worms that spread via email, disrupting businesses and critical services worldwide. Sobig.F was especially damaging, with global costs estimated at $30 billion.

WannaCry (2017)
• A ransomware attack that encrypted files on infected computers and demanded payment. It affected over 200,000 computers in 150 countries, causing more than $4 billion in damages, particularly to healthcare and government organizations.
Zeus (2007)
• A Trojan horse used to steal banking information and create botnets. It was responsible for massive financial theft and targeted Fortune 500 companies worldwide, with damages estimated in the billions.
Stuxnet (2010)
• Noted for its sophistication, Stuxnet was a cyberweapon designed to target Iran’s nuclear facilities. It was highly targeted and marked a new era in cyberwarfare.