My understanding is that a virus is an executable - and thus it needs to be executed to do anything. When you execute a virus (program), the process will be able to do whatever malicious thing it is supposed to.
I've been reading an answer, and this part:
Virus, on the other hand, need an executable image to spread within a system. They will spread to other executables within the same computer (they will attach themselves to those executables which become infected and able to spread the virus). And they spread to other computers as these executables are passed to them by any means (download, on a CD or floppy disk) and executed.
Spread to other executables? So if a virus spreads into my Photoshop executable, running Photoshop in the future will effectively execute the virus code? Essentially rendering my Photoshop program into a virus on itself?
I have never seen this sort of behaviour before - if it is true? (If yes I'll ask at SO how)