I was doing some research on Cordyceps earlier today (I was bored) and I found out that the fungus is actually regularly used for medicinal purposes, specifically in ciclosporin—a drug helpful in human organ transplants, since it suppresses the immune system. It's also used a lot in Asia, specifically in Tibet and China, in folk remedies, usually in the form of the Cordyceps mushroom.
If I had to guess as to the driving force behind the spread of this new fungus in the game, I would assume it had to do with the medicinal version of the fungus mutating and starting to infect humans. Of course, this is a little ridiculous in scientific terms since apparently the fungus' ability to modify its host's behavior evolved around 48 million years ago, so I would assume that if it didn't evolve to affect humans in all this time there is no reason for it to start affecting humans now. But then again it's a fictional story, so whatever's clever.
What I find more interesting is that apparently the fungus thrives in humid temperate and tropical forests, and the trailer definitely shows a city with an overgrowing flora. This leads me to believe that there might be areas of the United States in the game which are fungus-free, and Joel and Ellie's entire goal is to reach one of these cities. Maybe somewhere in the north, like Boston? Or maybe south to a desert region, like Texas? I dunno, just throwing out some ideas there.
At the very least I hope the origin and inner workings of this fungus are explored in the story.
