User blog:THEJJRAT/Theory: the Zombie Apocalypse(s) were caused by an alien invasion

isnt much of a theory because there's no evidence in existence but ok

In the main Gmod timeline (not including L4D), there has been a number of zombie apocalypses, and at least one alien invasion (if that episode was in universe). As most of these Gmod zombo videos have headcrabs, and facehuggers, I believe that sometime in the past or future an alien race invaded the planet and used parasites (headcrabs and xenomorphs) (and maybe tripods during the early stages) to cripple the human race. They also dispatched androids to Earth to wipe out the resistance. Said aliens might be the Mothership Zeta species, or the Combine because of transhuman Overwatch soldiers sometimes appearing in these videos (such as the End of the World adventure map).

The Left 4 Dead (assuming the Green Flu/non canon Mercy Hospital incident never happened in this universe) zombies may be from the planet that thisepisode took place on, if the species somehow got on Earth (spaceship that the L4D crew arrived on or the Toilet Toucher opening a portal). Or, the Left 4 Dead series was never canon in the first place and Johnny Toast never died.

It could've also been caused by the Toilet Toucher, as zombies (and shrimp monsters) reside in his basement. This may have just been test subjects, or survivors that took refugee in his basement and turned later on, Toilet never bothering to do anything about it.

Or
Remember that time when the multiverse started leaking out of that microwave and Maloney sacrificed himself to seal it? That same event happened again and that dimension that Johnny Ghost was trapped in (that had the zombies and crusty guy and barrels) leaked out enough to cause an outbreak.