How to write a Linux virus in 5 easy steps

Basic example, a bash virus runs, look for other bash scripts and inserts itself at the start of the existing script (eventually first checking if it was already infected). It may now check if it's time to do it's think ("rm -rf ~/*") or just exit and wait the user to make it run again by executing an infected script.

A worm runs by itself. Doesn't need a "host" program to run, but off course it needs some way of start running, and that can possibly be by adding itself to be called at the end of .bashrc/.profile/.whatever.

In the old days, the difference was that virus were writen in assembly, because they couldn't use something like libc, while worms, being a full programs, could be written in anything (and so, could be more powerful, like using system network libraries for infection).