In Brief: Differences Between a Virus, Worm, and Trojan Horse
Please keep in mind that these definitions may change as computer viruses become more complex. As of this revision, the computer virus authors are combining two or more viruses together to make one new virus. The new virus combines the features of each virus into one so that the virus will slip past the anti-virus software. It is very important to keep your anti-virus software updated.
Virus
A computer virus is a program that is designed to replicate itself and spread from file to file, usually attaching itself to applications. When this application is run, it can infect other files on a user's disk. By definition, human interaction is necessary for a virus to spread to another user's files. This can be performed by downloading files, trading diskettes with others, copying files to/from file servers, or sending email attachments. To combat a virus, be sure to use anti-virus software like McAfee's VirusScan or Norton Anti-virus from Symantec.
Worm
A computer worm also infects other computers, but it is spread to other computers on a network automatically and without the action of humans. This allows computer worms to spread more rapidly than computer viruses. A worm doesn't alter or delete files but instead they reside in memory, eat up system resources, and slow down your computer. To protect against a worm, networked users must keep up with operating system patches and updates and be aware of any suspicious traffic.
Trojan Horse
A Trojan Horse is a program that initially appears useful or benign and fools a user into running it. But while it runs, it could be allowing "back door" access to your computer by hackers or destroying files on your hard disk. For protection against a Trojan Horse, users must be suspicious of any unknown program and be sure it is safe before running it.
