What is computer virus? Explain different types of computer viruses.

A computer virus is a type of malicious software (malware) that attaches itself to legitimate programs or files and replicates when the host program is executed. It can corrupt data, steal information, or damage system files.

Types of computer viruses:

1. Boot Sector Virus — Infects the master boot record (MBR) of a hard disk. Activates when the computer boots up. Example: Brain virus.
2. File Infector Virus — Attaches to executable files (.exe, .com). Spreads when infected file is run. Example: Cascade virus.
3. Macro Virus — Infects documents that support macros (MS Word, Excel). Spreads through email attachments. Example: Melissa virus.
4. Polymorphic Virus — Changes its code each time it replicates, making it difficult for antivirus software to detect.
5. Trojan Horse — Disguises itself as legitimate software but performs malicious actions once installed.