Worms differ from computer viruses because they can propagate themselves, rather than using a carrier program or file. They simply create exact copies of themselves and use communication between computers to spread.
The Conficker worm is an example of an Internet worm that exploits a system vulnerability to infect machines over the network. Such worms are capable of spreading very rapidly, infecting large numbers of machines.
Some worms open a “back door” on the computer, allowing hackers to take control of it. Such computers can then be used to send spam mail (see Zombie).
Operating system vendors regularly issue patches to fix security loopholes in their software. To stay protected, turn on automatic updating to receive regular security updates for Windows or Apple machines.