What are some common types of computer viruses?

Computer viruses come in various forms, each with its unique way of infecting and damaging computers and data. Here are some common types:

1. File Infector Virus: This type of virus attaches itself to executable files (.exe or .com files, for example) and is activated when the infected file is run. It can then infect other executable files. Examples include Jerusalem and Cascade.

2. Macro Virus: These viruses target software that uses macros, like Microsoft Office applications. They are typically embedded in documents and activate when the document is opened, causing a sequence of actions to be executed automatically. Example: Mellisa

3. Boot Sector Virus: This type infects the boot sector of a hard drive or a removable disk (like a USB or a floppy disk in older computers). It activates when the computer starts up from the disk where the virus resides. Examples: Boot Sector Viruses 

4. Resident Virus: Resident viruses embed themselves in a computer’s memory. They can execute anytime when the operating system runs and infect files opened by the system. Example: Resident Virus

5. Direct Action Virus: This is a non-resident virus that acts when the file containing the virus is executed. It searches for other files to infect and typically attaches to files in the same directory as the virus. Example: Direct Action Virus

6. Polymorphic Virus: A sophisticated type, polymorphic viruses can change their code or signature patterns to evade detection by antivirus software. Example: Polymorphic Virus

7. Metamorphic Virus: Even more complex, these viruses rewrite their own code, making them very difficult to detect. They can change their form each time they infect a new file. Example: Metamorphic Virus

8. Web Scripting Virus: These exploit the code of web browsers and web pages. They usually come from malicious websites or by injecting code into legitimate websites. Example: Web Scripting Virus

9. Worms: Technically not a virus, but often lumped in with viruses, worms are self-replicating and spread across networks, exploiting vulnerabilities without requiring user interaction. Example: Worms

10. Trojan Horses: Also not a traditional virus, Trojans disguise themselves as legitimate software but perform malicious activities when executed. Example: Trojan Horse

Each type of virus has its unique characteristics and modes of infection, which can pose significant risks to computer systems, especially if they are not protected by up-to-date antivirus software.

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