What are computer viruses, worms, and Trojan horses? Are they any different from each other, given that these three terms are often used interchangeably? As a matter of fact, they are three distinct things, and should not be confused for one another.
The three do have something in common though: they are all malicious programs meant to cause damage to computers. Being able to distinguish among them allows users to better protect their devices.
This is characterised by its ability to propagate itself: it attaches to a file or program, such that it can travel from computer to computer, leaving in its wake infections. The severity of a computer virus will depend on the type thereof: some will have minimal effects while others can damage both software and hardware. Also, the majority of viruses only infect computers when the malicious program is run; so, they can exist on the computer without causing infection, and thus cannot be spread without a user action (like opening the file in question).
They are often spread when people share infected files and emails with viruses as attachments.
A worm is, in fact, a sub-category of a virus. The difference is that it can spread without human action. It travels by itself through file or information transport features on a computer system.
Also, a worm can replicate itself on a system such that a computer can send out multiple copies thereof: one can be sent from one computer to many more through an email address book.
If you think the virus and the worm are malicious, the Trojan Horse will make you reconsider things. This one is even worse. It can pass off as a seemingly-useful software, but you will find out about the extent of damage it can cause the moment you install or run it on your computer! Why would anyone fall for it, though? Well, it is often disguised as a legitimate software or file from an authentic source, which is why many people get tricked.
Some Trojans can actually be just for fun: they are more annoyance than trouble. However, some can be terribly damaging: they might do some serious damage by deleting files, or allowing other users have access to a system to extract confidential information.
Trojans differ from viruses and worms as they do not spread by infecting other files or by self-replicating.
The worst type: a combination
Some malicious individuals have merged the three types of malicious programs to produce a blended threat. This can be propagated via server and Internet vulnerabilities. It can spread in multiple ways, and the origin can be from several points. A blended threat is the worst of them all.