Around two hundred thousand years ago, a new disease appeared. No one had ever seen anything like it before. The odd thing is that those who are infected don't know they carry the condition. Even when told that they have contracted this disease, they either don't believe it, or continue about their lives and forget anything ever happened. Even after all this time, we have yet to find a cure, and the disease is now so wide spread that its horrific impact on our people has turned to a subtle way of life.
Those who contract the disease tend to age far more quickly than they should, dying prematurely. Some even become delusional and take their own lives. Because of the primitive fear of death that they develop from the illness, most of them create stories, believing that after death they will rise up and live again. Some of them even kill one another, for any reason. For whatever reason they never touch the uninfected, only their own kind.
The above are symptoms common to most, if not all of them. However, there are symptoms to the disease which only spread to a small percent of their population. For example, some see things that aren't there. Others continually ingest different poisons, claiming that they "need" them. A few have been known to even kill their own families. Even these behaviors are just the tip of the ice berg.
Most of these infected individuals don't pay us any attention, even when physically assaulted by us they only respond in screams and choppy sentences, running to others of their kind seeking help. When left untouched, it's almost as if they can't even see us. And even though we have yet to find a cure, and most of us doubt we ever will, we have come up with a name for the disease. Humanity.