How can it be that there are so many left in the world who don't believe the Holocaust happened?

I just don't understand it. Even my own grandma is on this list. How did this happen. I don't understand.<br><br><br>We had a left leaning professor on campus who denied the holohoax event. Some say he was an anti Semite. But I don't understand what drives someone to think and say things like this and not take into account the proof and facts. I just don't get people who deny the truth.