As the security team at Symantec notes, viruses are nasty computer programs designed to travel from file to file within a single computer. Worms, on the other hand, are specifically engineered to travel between computers.

This isn't to say that viruses can't travel between computers. Much like a burr on bear fur, viruses hop rides on email attachments sent by humans. Worms, however, are much nastier -- they don't need host programs to propagate.

A really bad worm can infect thousands of passively networked computers in a matter of hours. Worms may not damage the individual computers, but they can definitely jam up a network. Fortunately, they afflict more network administrators than home users.

Another variant of computer nasty is the Trojan horse. These pernicious programs are written to disguise their damage. Trojan horses can also carry hidden spyware, so watch out, file traders. It's a dangerous world out there -- safety pays in many ways.

The most common blunder people make when the topic of a computer virus arises is to refer to a worm or Trojan horse as a virus. While the words Trojan, worm and virus are often used interchangeably, they are not exactly the same. Viruses, worms and Trojan Horses are all malicious programs that can cause damage to your computer, but there are differences among the three, and knowing those differences can help you to better protect your computer from their often damaging effects.

What Is a Virus?

A computer virus attaches itself to a program or file enabling it to spread from one computer to another, leaving infections as it travels. Like a human virus, a computer virus can range in severity: some may cause only mildly annoying effects while others can damage your hardware, software or files.

Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it actually cannot infect your computer unless you run or open the malicious program. It is important to note that a virus cannot be spread without a human action, (such as running an infected program) to keep it going. 

People continue the spread of a computer virus, mostly unknowingly, by sharing infecting files or sending e-mails with viruses as attachments in the e-mail.

What Is a Worm?

A worm is similar to a virus by design and is considered to be a sub-class of a virus. Worms spread from computer to computer, but unlike a virus, it has the capability to travel without any human action. A worm takes advantage of file or information transport features on your system, which is what allows it to travel unaided.

The biggest danger with a worm is its capability to replicate itself on your system, so rather than your computer sending out a single worm, it could send out hundreds or thousands of copies of itself, creating a huge devastating effect. One example would be for a worm to send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates and sends itself out to everyone listed in each of the receiver's address book, and the manifest continues on down the line. 

Due to the copying nature of a worm and its capability to travel across networks the end result in most cases is that the worm consumes too much system memory (or network bandwidth), causing Web servers, network servers and individual computers to stop responding. In recent worm attacks such as the much-talked-about Blaster Worm, the worm has been designed to tunnel into your system and allow malicious users to control your computer remotely.