remember...it still needs a compiler?
- Agreeing further. Most of the viruses are either executable or attached to executables, in a way it cannot be just a source code, as far as I can elaborate, like any other program, a virus has to be a compiled version of the source or it relies to a program (i.e. an executable, dll, registry) to work or spread.

I mean since assembly is based on the commands used for the specific processor of a computer
- You are missing the link here, what you mean here is the raw material of the product, the product itself (VIRUS) had been the fact that it was assembled, tested, and deployed should be the final product in which what nihil had pointed out to be 'COMPILED'.

* Ow, someone will throw a bomb on me about 'SCRIPTS' which are actually source code based, keep in mind that malicious scripts, macros, although act as viruses still rely on programs like browsers and spreadsheet progs to spread

A nice read about viruses and hows - From Wikipedia - http://en.wikipedia.org/wiki/Computer_virus

-GONE