In the book (originally a radio play) the name of the disease is Acquired Metastructural Pediculosis (AMPS), and is caused by a virus. The author of Pontypool was a linguist, and threw together a bunch of known conditions: (aphasia, phonemic paraphasias, echolalia, semantic paraphasias, anomia, verbal stereotypies, etc.) and associated them with his storyās disease. The interesting twist is that rather than a physical virus (a single strand of genetic information encased within a protein capsule, and transmitted physically) the virus is multivariate strand(s) of non-genetic information transmitted within sound waves. (Although not explicitly mentioned, the term for this is āmemeā, or āmemeticsā, in which the emerging study is named - a branch of metaphysics.)
Itās more complex than a traditional virus, in that itās not bound - there is no capsule of ābadnessā that moves from one person to another; itās not self-contained. The transmitted/received trigger is only a component of the virus, and serves as a key to unlock components already contained within the host. Further ā words/sounds themselves have different meanings to different people. So the key for one is not necessarily the key for another. All this leads to seeming inconsistency in how the virus is transmitted and manifests (but the Dr. tells us it always ends up the same.) The puzzle of the story is figuring out how it transmits, and then how to inoculate it.
The notion of it being a terrorist attack on English speaking Canadians (presumably by French speaking Canadians) is supported (at least in the minds of the characters), and it is not insignificant that the Laurel-Ann character fought in Afghanistan ā but one way to look at that is to think that within the English speaking people a meme-virus may evolve itself that causes the malfunction. So it can be taken both ways: āsomeoneā is attacking the English speakers ā or ā the English speakers innately are susceptible to this virus and go crazy in a way that scales up, like an epidemic, leading to war. (i.e. religion ā itself a meme complex, and thus religious wars. The studio is, after all, broadcast from an old church.)
The kicker is that the entire story may actually be within a single infected person ā a kind of daydream. So all characters are contained within his head ā the radio studio. All the things outside (that we never see) are full abstractions. (The story is gloriously polysemous!!)
So where do physical viruses come from? They come from the dawn of life on earth, and in fact may predate cellular life, and have evolved along with us. We need them to survive. Occasionally one mutates and evolves into an epidemic. Same with this memetic virus AMPS.
So just like SARS originated in Chinese bats, who infected civets, who made it to a Chinese food market. I think the Pontypool virus was originated in the opening dialogue of the movie, which was recorded the day before (and played the day before) and was being rebroadcast in the car as Mazzy was driving to work in the early AM. Out of the chaos of that speech ā the memetic virus was born. That is my simplistic understanding of āthe bigger pictureā behind it all that you requested.