From what I understand, the biggest and probably only line of defense against unauthorized software (viruses and worms) from being installed on the system is to simply not allow it unless an admin password has been given.
But what about trojan horses? Let's say a human manages to hack into a trusted PPA repository, and infects various legit packages with viruses (for the sake of the point - someone infects f.lux or wine with a virus that plays a huge looping Nyan Cat on the top layers of the screen with extra loud music). The next user who downloads the package gets infected, because they allowed the program to be installed. They never thought of the possibility of a trojan horse.
The trojan is likely removed quickly by one of the many thousands of eyes watching the Linux community, but chances are good that at least some users get infected. The definition of a virus is something that installs itself on a computer system without consent of an authorized human, and reproduces itself by infecting more systems in that manner. If this happens, they have a virus.
Is that an argument against the supposedly impenetrable security of Linux?