What caused the zombie apocalypse in The Last Of Us?

There are a number of theories as far as what caused the zombie apocalypse. One is that it was a virus outbreak. That makes a lot of sense and many cling to that believe because the exact cause may never actually be known in its entirety. Another thought is that the parasites killed their hosts. No matter the exact underlying cause, the end result was a virus indeed. It was transitted through the air, water, sexual contact, and by many other ways too.

In reality, it is possible for things to spread in those types of ways. Germs, viruses, and even chemical warfare could all be contagious through a number of ways very similar to these. While the though of a zombie apacalypse is scary to think about, it is purely fiction. Or...is it?