Q. Is there a difference between a computer virus and a Trojan horse?
A. The terms are sometimes used interchangeably, but they are not the same.
A computer virus attaches itself to a file or program and replicates itself to spread from one computer to another or from file to file. A virus may be present in your computer but not infect it unless you take a specific action, such as running a particular program.
Likewise, a specific action by you, such as sending email with infected attachments, is required for the virus to make its rounds to other machines. Once it makes its presence known, the effect can range from annoying messages flashed on your screen to serious damage done to hardware, software or files.
A Trojan horse does not replicate. It is sent out by a hacker, usually in spam emails, and installs itself on your computer when you click on a link or attachment.
Often disguised as a legitimate file or a program such as a computer game, a Trojan can, like a virus, be mildly bothersome or outright destructive. It can also provide cyber-crooks with remote access to your computer, giving them user names, passwords and financial account numbers.
Because of technicalities concerning their programming code, viruses are likely to be detected by your computer's antivirus software even before they are individually identified by antivirus experts.
Trojans can be harder to detect, unless they've been specifically identified. That means it's especially important to check regularly for protection program updates, which contain information on newly identified Trojans.
Yet another kind of malicious software is known as a worm. It's similar to a virus in that it spreads from computer to computer. But it can proliferate and travel without help of any human action.
A worm can replicate on your computer, sending out hundreds or thousands of copies of itself. It might do this by attaching itself to emails it sends to everyone in your email address book and then traveling to everyone in the address books of those recipients.
Sid Kirchheimer writes about consumer and health issues. Have a question for Sid Kirchheimer about a new product, a new kind of bank account? Check out the Ask Sid archive. If you don’t find your answer there, send a query.