Do You Know Your Digital Vermin? Of Viruses, Worms, and Trojans... Vangie Beal
The most common blunder when the topic of a computer virus arises is that people will often refer to a worm or Trojan Horse as a virus. While the words Trojan, worm, and virus are used interchangeably, they are not the same. Viruses, worms, and Trojan Horses are all malicious programs that can cause damage to your computer, but there are differences between the three, and knowing those differences can help you to better protect your computer from their often-damaging effects.
Viruses
A computer virus attaches itself to a program or file so it can spread from one computer to another, leaving infections as it travels. Much like human viruses, computer viruses can range in severity; some viruses cause only mildly annoying effects while others can damage your hardware, software applications, or files.
Almost all viruses are attached to an executable file, which means the virus may exist on your computer but can't infect your computer unless you run – or open – the malicious program. It's important to note that a virus can't 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.
Worms
A worm is similar to a virus by its design, and is considered to be a sub-class of a virus. Worms spread from computer to computer, but unlike a virus, they have the ability to travel without any help from a person. A worm takes advantage of file or information transport features on your system, which allow it to travel unaided.
The biggest danger with a worm is its ability to replicate itself on your system, so rather than your computer sending a single worm back out into the wild, it could actually 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. The worm copies would then replicate and send themselves out to everyone listed in each of the receivers' address books — and so the manifest continues on down the line.
Due to the copying nature of a worm and its ability 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 more recent 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.
Trojans
The digital Trojan Horse of today is as full of trickery as the mythological Trojan Horse after which it was named. At first glance a Trojan appears to be useful software but will actually do damage once installed or run on your computer.
People on the receiving end of a Trojan are usually tricked into opening them because they appear to be receiving legitimate software or files from a legitimate source. When a Trojan activates on your computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons, etc.), while others can cause serious damage by deleting files and destroying information on your system.
Trojans can also create a backdoor to your computer that gives malicious users access to your system, possibly compromising confidential or personal information. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.
