A worm is a malicious program that is placed on a computer and then activated. When a worm is activated, it executes a program that is designed to cause some kind of harm to the computer or user using the computer. When the worms starts to work it replicates itself and carries out its function. 

Destructive worms:
  • MyDoom (2004) - $38 Billion 
  • ILOVEYOU (2000) - $15 Billion
  • Conficker (2007) - $9.1 Billion