There are many different types of computer viruses. Most viruses work as Trojan horses, which means they present themselves as legitimate software and the user unwittingly installs them on his or her machine. Worms are a sub-class of computer viruses. Worms are defined as viruses that have the ability to spread on their own using a PC's internet transport protocol, rather than being spread by user actions.
Most viruses, like the Trojan horse, are dependant on a user to actually spread the virus from machine to machine - you send the application to someone, thinking it is legitimate, or actively download and install it. Worms, on the other hand, have to unique ability to spread on their own without any user input at all. What makes worms dangerous are that once a PC in infected the worm can replicate itself many times within the PC and infect many other machines.
A common virus worm works by exposing security loopholes in the Microsoft windows operating system, and also the Microsoft Outlook email software, which of course is the most common email software used in offices. A typical virus worm will be received by the user in the form of an email with a subject line that disguises it as something normal and when the user opens the email, the worm spreads throughout the system and sends itself to other users on the infected users outlook contact list.
One of the more famous internet worms was the so called 'melissa' worms, which spread extremely rapidly throughout the world through email. What makes these worms so effective is that they exploit the users own contact list to spread and send out more emails. When the worm spreads, it sends emails that in many cases are coming from someone the recipient knows: being on the contact list, the user is not suspicious when opening an email from a friend with the subject line "Hi." When they open the email, there is usually no message, but it still seems to the user as if nothing has happened. Meanwhile the worms have spread throughout the system and are already sending out emails to infect other users.
Because of the exponential nature of worms, they can debilitate networks quickly. Especially in a corporate environment, where many employees may have the entire company on their contact lists, worms can quickly shut down a company's email server or entire network, causing massive damage in terms of productivity.
Worm removal can be difficult once it has infected a large internal network, like that of an office or institution. For the home user, worm removal is easier and most worms can be removed by all of the most common antivirus packages. Like all virus protection, the most important thing the average user can do to protect him or herself from worms is to have antivirus software installed, running, and up to date. Once this is accomplished, worms should pose little to no threat, as common antivirus software like Symantec antivirus is capable of scanning incoming and outgoing emails from Microsoft outlook, which is how most worms spread.