From nakedsecurity.sophos.com
OK, technically, this article is about malware in general, not about viruses in particular.
Strictly speaking, virus refers to a type of malware that spreads by itself, so that once it’s in your system, you may end up with hundreds or even thousands of infected files…
…on every computer in your network, and in the networks your network can see, and so on, and so on.
These days, however, the crooks don’t really need to program auto-spreading into their malware – thanks to always-on internet connectivity, the “spreading” part is easier than ever, so that’s one attention-grabbing step the crooks no longer need to use.