Some people consider a worm to be a sub-class of a virus because it spreads from computer to computer. Unlike a virus though, a worm does not need a person to spread it. Worms spread by exploiting vulnerabilities in operating systems. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole and then starts replicating from there. A worm also spreads itself using transport methods that files use, such as email.

Worms can infiltrate email programs and send replicates of themselves to everyone in the address book. Once it infects those computers, it repeats the pattern by infiltrating those email programs and sending replicates to everyone in those address books. Thus worms can spread very quickly and impact networks, websites, and the Internet since it is increasing traffic as it sends itself across the globe.

Whereas a user can minimize exposure to viruses by practicing good user security practices such as never opening an email attachment unless the user requested it from the sender, a worm cannot be thwarted by such practices.

Protecting computers against worms is similar to as for viruses. The best way to protect a computer against worm is to install anti-virus software and keep it updated. It should automatically scan all incoming and outgoing email messages. It should also regularly scan every file on the computer to ensure that the worm is not already installed on the computer. A firewall should be installed. Since worms exploit vulnerabilities in operating systems, the operating system of the computer should be regularly updated. Browsers should also be updated.