What are the effect and causes of Computer Viruses?

Computer virus: It is a computer program that replicates(multiplies) itself and destroys data, files, and programs of our computer system. Types of computer viruses
  1. Boot sector: Infects master boot record (MBR) of our computer system. (Booting refers to starting of computer system)
  2. File infector or program infector virus: Infect programs or executable files (.exe, .dmg, installer files)
  3. Multipartite virus: Infects both boot sector and application programs.
  4. Macro virus: Infects certain files created using macros such as MS Word, MS Excel, etc
  5. Stealth virus: This can hide from being detected. It's hard to detect
  6. Trojan horse: They do not replicate but are installed once then it starts malfunctioning.
  7. WORM: Write Once Read Many
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