What’s the difference between viruses, trojans and worms?

They all sound like human diseases, but if you’re a computer the consequences can be just a devastating.

Asked by: Simon Rankin, Maidstone


A virus is a nasty piece of software that is inserted into a normal piece of software, just as a biological virus infects a cell. When the normal software is run, the virus copies itself into other software while also doing unwanted things such as recording keystrokes to steal passwords.

A trojan is similar, but it does not replicate itself. It hides inside a seemingly innocuous program – run the program and the trojan wreaks havoc, from deleting your files to giving hackers access to your system.

A worm is an independent program that replicates on its own, typically spreading across networks and causing major disruption to systems.


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